Request & Response Objects

The request and response objects provide an abstraction around HTTP requests and responses. The request object in CakePHP allows you to introspect an incoming request, while the response object allows you to effortlessly create HTTP responses from your controllers.


class Cake\Http\ServerRequest

ServerRequest is the default request object used in CakePHP. It centralizes a number of features for interrogating and interacting with request data. On each request one Request is created and then passed by reference to the various layers of an application that use request data. By default the request is assigned to $this->request, and is available in Controllers, Cells, Views and Helpers. You can also access it in Components using the controller reference.

Changed in version 4.4.0: The ServerRequest is available via DI. So you can get it from container or use it as a dependency for your service.

Some of the duties ServerRequest performs include:

  • Processing the GET, POST, and FILES arrays into the data structures you are familiar with.

  • Providing environment introspection pertaining to the request. Information like the headers sent, the client’s IP address, and the subdomain/domain names the server your application is running on.

  • Providing access to request parameters both as array indexes and object properties.

CakePHP’s request object implements the PSR-7 ServerRequestInterface making it easier to use libraries from outside of CakePHP.

Request Parameters

The request exposes routing parameters through the getParam() method:

$controllerName = $this->request->getParam('controller');

To get all routing parameters as an array use getAttribute():

$parameters = $this->request->getAttribute('params');

All Route Elements are accessed through this interface.

In addition to Route Elements, you also often need access to Passed Arguments. These are both available on the request object as well:

// Passed arguments
$passedArgs = $this->request->getParam('pass');

Will all provide you access to the passed arguments. There are several important/useful parameters that CakePHP uses internally, these are also all found in the routing parameters:

  • plugin The plugin handling the request. Will be null when there is no plugin.

  • controller The controller handling the current request.

  • action The action handling the current request.

  • prefix The prefix for the current action. See Prefix Routing for more information.

Query String Parameters

Cake\Http\ServerRequest::getQuery($name, $default = null)

Query string parameters can be read using the getQuery() method:

// URL is /posts/index?page=1&sort=title
$page = $this->request->getQuery('page');

You can either directly access the query property, or you can use getQuery() method to read the URL query array in an error-free manner. Any keys that do not exist will return null:

$foo = $this->request->getQuery('value_that_does_not_exist');
// $foo === null

// You can also provide default values
$foo = $this->request->getQuery('does_not_exist', 'default val');

If you want to access all the query parameters you can use getQueryParams():

$query = $this->request->getQueryParams();

Request Body Data

Cake\Http\ServerRequest::getData($name, $default = null)

All POST data normally available through PHP’s $_POST global variable can be accessed using Cake\Http\ServerRequest::getData(). For example:

// An input with a name attribute equal to 'title' is accessible at
$title = $this->request->getData('title');

You can use a dot separated names to access nested data. For example:

$value = $this->request->getData('address.street_name');

For non-existent names the $default value will be returned:

$foo = $this->request->getData('value.that.does.not.exist');
// $foo == null

You can also use Body Parser Middleware to parse request body of different content types into an array, so that it’s accessible through ServerRequest::getData().

If you want to access all the data parameters you can use getParsedBody():

$data = $this->request->getParsedBody();

File Uploads

Uploaded files can be accessed through the request body data, using the Cake\Http\ServerRequest::getData() method described above. For example, a file from an input element with a name attribute of attachment, can be accessed like this:

$attachment = $this->request->getData('attachment');

By default file uploads are represented in the request data as objects that implement \Psr\Http\Message\UploadedFileInterface. In the current implementation, the $attachment variable in the above example would by default hold an instance of \Laminas\Diactoros\UploadedFile.

Accessing the uploaded file details is fairly simple, here’s how you can obtain the same data as provided by the old style file upload array:

$name = $attachment->getClientFilename();
$type = $attachment->getClientMediaType();
$size = $attachment->getSize();
$tmpName = $attachment->getStream()->getMetadata('uri');
$error = $attachment->getError();

Moving the uploaded file from its temporary location to the desired target location, doesn’t require manually accessing the temporary file, instead it can be easily done by using the objects moveTo() method:


In an HTTP environment, the moveTo() method will automatically validate whether the file is an actual uploaded file, and throw an exception in case necessary. In an CLI environment, where the concept of uploading files doesn’t exist, it will allow to move the file that you’ve referenced irrespective of its origins, which makes testing file uploads possible.


Returns the uploaded file at a specific path. The path uses the same dot syntax as the Cake\Http\ServerRequest::getData() method:

$attachment = $this->request->getUploadedFile('attachment');

Unlike Cake\Http\ServerRequest::getData(), Cake\Http\ServerRequest::getUploadedFile() would only return data when an actual file upload exists for the given path, if there is regular, non-file request body data present at the given path, then this method will return null, just like it would for any non-existent path.


Returns all uploaded files in a normalized array structure. For the above example with the file input name of attachment, the structure would look like:

      'attachment' => object(Laminas\Diactoros\UploadedFile) {
          // ...
Cake\Http\ServerRequest::withUploadedFiles(array $files)

This method sets the uploaded files of the request object, it accepts an array of objects that implement \Psr\Http\Message\UploadedFileInterface. It will replace all possibly existing uploaded files:

$files = [
    'MyModel' => [
        'attachment' => new \Laminas\Diactoros\UploadedFile(
        'anotherAttachment' => new \Laminas\Diactoros\UploadedFile(

$this->request = $this->request->withUploadedFiles($files);


Uploaded files that have been added to the request via this method, will not be available in the request body data, ie you cannot retrieve them via Cake\Http\ServerRequest::getData()! If you need them in the request data (too), then you have to set them via Cake\Http\ServerRequest::withData() or Cake\Http\ServerRequest::withParsedBody().


Cake\Http\ServerRequest::input($callback[, $options])

When building REST services, you often accept request data on PUT and DELETE requests. Any application/x-www-form-urlencoded request body data will automatically be parsed and set to $this->data for PUT and DELETE requests. If you are accepting JSON or XML data, see below for how you can access those request bodies.

When accessing the input data, you can decode it with an optional function. This is useful when interacting with XML or JSON request body content. Additional parameters for the decoding function can be passed as arguments to input():

$jsonData = $this->request->input('json_decode');

Environment Variables (from $_SERVER and $_ENV)

Cake\Http\ServerRequest::putenv($key, $value = null)

ServerRequest::getEnv() is a wrapper for getenv() global function and acts as a getter/setter for environment variables without having to modify globals $_SERVER and $_ENV:

// Get the host
$host = $this->request->getEnv('HTTP_HOST');

// Set a value, generally helpful in testing.
$this->request->withEnv('REQUEST_METHOD', 'POST');

To access all the environment variables in a request use getServerParams():

$env = $this->request->getServerParams();

XML or JSON Data

Applications employing REST often exchange data in non-URL-encoded post bodies. You can read input data in any format using input(). By providing a decoding function, you can receive the content in a deserialized format:

// Get JSON encoded data submitted to a PUT/POST action
$jsonData = $this->request->input('json_decode');

Some deserializing methods require additional parameters when called, such as the ‘as array’ parameter on json_decode. If you want XML converted into a DOMDocument object, input() supports passing in additional parameters as well:

// Get XML encoded data submitted to a PUT/POST action
$data = $this->request->input('Cake\Utility\Xml::build', ['return' => 'domdocument']);

Path Information

The request object also provides useful information about the paths in your application. The base and webroot attributes are useful for generating URLs, and determining whether or not your application is in a subdirectory. The attributes you can use are:

// Assume the current request URL is /subdir/articles/edit/1?page=1

// Holds /subdir/articles/edit/1?page=1
$here = $request->getRequestTarget();

// Holds /subdir
$base = $request->getAttribute('base');

// Holds /subdir/
$base = $request->getAttribute('webroot');

Checking Request Conditions

Cake\Http\ServerRequest::is($type, $args...)

The request object provides a way to inspect certain conditions in a given request. By using the is() method you can check a number of common conditions, as well as inspect other application specific request criteria:

$isPost = $this->request->is('post');

You can also extend the request detectors that are available, by using Cake\Http\ServerRequest::addDetector() to create new kinds of detectors. There are different types of detectors that you can create:

  • Environment value comparison - Compares a value fetched from env() for equality with the provided value.

  • Header value comparison - If the specified header exists with the specified value, or if the callable returns true.

  • Pattern value comparison - Pattern value comparison allows you to compare a value fetched from env() to a regular expression.

  • Option based comparison - Option based comparisons use a list of options to create a regular expression. Subsequent calls to add an already defined options detector will merge the options.

  • Callback detectors - Callback detectors allow you to provide a ‘callback’ type to handle the check. The callback will receive the request object as its only parameter.

Cake\Http\ServerRequest::addDetector($name, $options)

Some examples would be:

// Add an environment detector.
    ['env' => 'REQUEST_METHOD', 'value' => 'POST']

// Add a pattern value detector.
    ['env' => 'HTTP_USER_AGENT', 'pattern' => '/iPhone/i']

// Add an option detector
$this->request->addDetector('internalIp', [
    'env' => 'CLIENT_IP',
    'options' => ['', '']

// Add a header detector with value comparison
$this->request->addDetector('fancy', [
    'env' => 'CLIENT_IP',
    'header' => ['X-Fancy' => 1]

// Add a header detector with callable comparison
$this->request->addDetector('fancy', [
    'env' => 'CLIENT_IP',
    'header' => ['X-Fancy' => function ($value, $header) {
        return in_array($value, ['1', '0', 'yes', 'no'], true);

// Add a callback detector. Must be a valid callable.
    function ($request) {
        return $request->getParam('awesome');

// Add a detector that uses additional arguments.
        'accept' => ['text/csv'],
        'param' => '_ext',
        'value' => 'csv',

There are several built-in detectors that you can use:

  • is('get') Check to see whether the current request is a GET.

  • is('put') Check to see whether the current request is a PUT.

  • is('patch') Check to see whether the current request is a PATCH.

  • is('post') Check to see whether the current request is a POST.

  • is('delete') Check to see whether the current request is a DELETE.

  • is('head') Check to see whether the current request is HEAD.

  • is('options') Check to see whether the current request is OPTIONS.

  • is('ajax') Check to see whether the current request came with X-Requested-With = XMLHttpRequest.

  • is('ssl') Check to see whether the request is via SSL.

  • is('flash') Check to see whether the request has a User-Agent of Flash.

  • is('json') Check to see whether the request URL has ‘json’ extension or the Accept header is set to ‘application/json’.

  • is('xml') Check to see whether the request URL has ‘xml’ extension or the Accept header is set to ‘application/xml’ or ‘text/xml’.

ServerRequest also includes methods like Cake\Http\ServerRequest::domain(), Cake\Http\ServerRequest::subdomains() and Cake\Http\ServerRequest::host() to make applications that use subdomains simpler.

Session Data

To access the session for a given request use the getSession() method or use the session attribute:

$session = $this->request->getSession();
$session = $this->request->getAttribute('session');

$data = $session->read('sessionKey');

For more information, see the Sessions documentation for how to use the session object.

Host and Domain Name

Cake\Http\ServerRequest::domain($tldLength = 1)

Returns the domain name your application is running on:

// Prints ''
echo $request->domain();
Cake\Http\ServerRequest::subdomains($tldLength = 1)

Returns the subdomains your application is running on as an array:

// Returns ['my', 'dev'] for ''
$subdomains = $request->subdomains();

Returns the host your application is on:

// Prints ''
echo $request->host();

Reading the HTTP Method


Returns the HTTP method the request was made with:

// Output POST
echo $request->getMethod();

Restricting Which HTTP method an Action Accepts


Set allowed HTTP methods. If not matched, will throw MethodNotAllowedException. The 405 response will include the required Allow header with the passed methods:

public function delete()
    // Only accept POST and DELETE requests
    $this->request->allowMethod(['post', 'delete']);

Reading HTTP Headers

Allows you to access any of the HTTP_* headers that were used for the request. For example:

// Get the header as a string
$userAgent = $this->request->getHeaderLine('User-Agent');

// Get an array of all values.
$acceptHeader = $this->request->getHeader('Accept');

// Check if a header exists
$hasAcceptHeader = $this->request->hasHeader('Accept');

While some apache installs don’t make the Authorization header accessible, CakePHP will make it available through apache specific methods as required.

Cake\Http\ServerRequest::referer($local = true)

Returns the referring address for the request.


Returns the current visitor’s IP address.

Trusting Proxy Headers

If your application is behind a load balancer or running on a cloud service, you will often get the load balancer host, port and scheme in your requests. Often load balancers will also send HTTP-X-Forwarded-* headers with the original values. The forwarded headers will not be used by CakePHP out of the box. To have the request object use these headers set the trustProxy property to true:

$this->request->trustProxy = true;

// These methods will now use the proxied headers.
$port = $this->request->port();
$host = $this->request->host();
$scheme = $this->request->scheme();
$clientIp = $this->request->clientIp();

Once proxies are trusted the clientIp() method will use the last IP address in the X-Forwarded-For header. If your application is behind multiple proxies, you can use setTrustedProxies() to define the IP addresses of proxies in your control:

$request->setTrustedProxies(['', '']);

After proxies are trusted clientIp() will use the first IP address in the X-Forwarded-For header providing it is the only value that isn’t from a trusted proxy.

Checking Accept Headers

Cake\Http\ServerRequest::accepts($type = null)

Find out which content types the client accepts, or check whether it accepts a particular type of content.

Get all types:

$accepts = $this->request->accepts();

Check for a single type:

$acceptsJson = $this->request->accepts('application/json');
Cake\Http\ServerRequest::acceptLanguage($language = null)

Get all the languages accepted by the client, or check whether a specific language is accepted.

Get the list of accepted languages:

$acceptsLanguages = $this->request->acceptLanguage();

Check whether a specific language is accepted:

$acceptsSpanish = $this->request->acceptLanguage('es-es');

Reading Cookies

Request cookies can be read through a number of methods:

// Get the cookie value, or null if the cookie is missing.
$rememberMe = $this->request->getCookie('remember_me');

// Read the value, or get the default of 0
$rememberMe = $this->request->getCookie('remember_me', 0);

// Get all cookies as an hash
$cookies = $this->request->getCookieParams();

// Get a CookieCollection instance
$cookies = $this->request->getCookieCollection()

See the Cake\Http\Cookie\CookieCollection documentation for how to work with cookie collection.

Uploaded Files

Requests expose the uploaded file data in getData() or getUploadedFiles() as UploadedFileInterface objects:

// Get a list of UploadedFile objects
$files = $request->getUploadedFiles();

// Read the file data.

// Move the file.

Manipulating URIs

Requests contain a URI object, which contains methods for interacting with the requested URI:

// Get the URI
$uri = $request->getUri();

// Read data out of the URI.
$path = $uri->getPath();
$query = $uri->getQuery();
$host = $uri->getHost();


class Cake\Http\Response

Cake\Http\Response is the default response class in CakePHP. It encapsulates a number of features and functionality for generating HTTP responses in your application. It also assists in testing, as it can be mocked/stubbed allowing you to inspect headers that will be sent.

Response provides an interface to wrap the common response-related tasks such as:

  • Sending headers for redirects.

  • Sending content type headers.

  • Sending any header.

  • Sending the response body.

Dealing with Content Types

Cake\Http\Response::withType($contentType = null)

You can control the Content-Type of your application’s responses with Cake\Http\Response::withType(). If your application needs to deal with content types that are not built into Response, you can map them with setTypeMap() as well:

// Add a vCard type
$this->response->setTypeMap('vcf', ['text/v-card']);

// Set the response Content-Type to vcard.
$this->response = $this->response->withType('vcf');

Usually, you’ll want to map additional content types in your controller’s beforeFilter() callback, so you can leverage the automatic view switching features of RequestHandlerComponent if you are using it.

Sending Files

Cake\Http\Response::withFile(string $path, array $options = [])

There are times when you want to send files as responses for your requests. You can accomplish that by using Cake\Http\Response::withFile():

public function sendFile($id)
    $file = $this->Attachments->getFile($id);
    $response = $this->response->withFile($file['path']);
    // Return the response to prevent controller from trying to render
    // a view.
    return $response;

As shown in the above example, you must pass the file path to the method. CakePHP will send a proper content type header if it’s a known file type listed in Cake\Http\Response::$_mimeTypes. You can add new types prior to calling Cake\Http\Response::withFile() by using the Cake\Http\Response::withType() method.

If you want, you can also force a file to be downloaded instead of displayed in the browser by specifying the options:

$response = $this->response->withFile(
    ['download' => true, 'name' => 'foo']

The supported options are:


The name allows you to specify an alternate file name to be sent to the user.


A boolean value indicating whether headers should be set to force download.

Sending a String as File

You can respond with a file that does not exist on the disk, such as a pdf or an ics generated on the fly from a string:

public function sendIcs()
    $icsString = $this->Calendars->generateIcs();
    $response = $this->response;

    // Inject string content into response body
    $response = $response->withStringBody($icsString);

    $response = $response->withType('ics');

    // Optionally force file download
    $response = $response->withDownload('filename_for_download.ics');

    // Return response object to prevent controller from trying to render
    // a view.
    return $response;

Setting Headers

Cake\Http\Response::withHeader($header, $value)

Setting headers is done with the Cake\Http\Response::withHeader() method. Like all of the PSR-7 interface methods, this method returns a new instance with the new header:

// Add/replace a header
$response = $response->withHeader('X-Extra', 'My header');

// Set multiple headers
$response = $response->withHeader('X-Extra', 'My header')
    ->withHeader('Location', '');

// Append a value to an existing header
$response = $response->withAddedHeader('Set-Cookie', 'remember_me=1');

Headers are not sent when set. Instead, they are held until the response is emitted by Cake\Http\Server.

You can now use the convenience method Cake\Http\Response::withLocation() to directly set or get the redirect location header.

Setting the Body


To set a string as the response body, do the following:

// Set a string into the body
$response = $response->withStringBody('My Body');

// If you want a json response
$response = $response->withType('application/json')
    ->withStringBody(json_encode(['Foo' => 'bar']));

To set the response body, use the withBody() method, which is provided by the Laminas\Diactoros\MessageTrait:

$response = $response->withBody($stream);

Be sure that $stream is a Psr\Http\Message\StreamInterface object. See below on how to create a new stream.

You can also stream responses from files using Laminas\Diactoros\Stream streams:

// To stream from a file
use Laminas\Diactoros\Stream;

$stream = new Stream('/path/to/file', 'rb');
$response = $response->withBody($stream);

You can also stream responses from a callback using the CallbackStream. This is useful when you have resources like images, CSV files or PDFs you need to stream to the client:

// Streaming from a callback
use Cake\Http\CallbackStream;

// Create an image.
$img = imagecreate(100, 100);
// ...

$stream = new CallbackStream(function () use ($img) {
$response = $response->withBody($stream);

Setting the Character Set


Sets the charset that will be used in the response:

$this->response = $this->response->withCharset('UTF-8');

Interacting with Browser Caching


You sometimes need to force browsers not to cache the results of a controller action. Cake\Http\Response::withDisabledCache() is intended for just that:

public function index()
    // Disable caching
    $this->response = $this->response->withDisabledCache();


Disabling caching from SSL domains while trying to send files to Internet Explorer can result in errors.

Cake\Http\Response::withCache($since, $time = '+1 day')

You can also tell clients that you want them to cache responses. By using Cake\Http\Response::withCache():

public function index()
    // Enable caching
    $this->response = $this->response->withCache('-1 minute', '+5 days');

The above would tell clients to cache the resulting response for 5 days, hopefully speeding up your visitors’ experience. The withCache() method sets the Last-Modified value to the first argument. Expires header and the max-age directive are set based on the second parameter. Cache-Control’s public directive is set as well.

Fine Tuning HTTP Cache

One of the best and easiest ways of speeding up your application is to use HTTP cache. Under this caching model, you are only required to help clients decide if they should use a cached copy of the response by setting a few headers such as modified time and response entity tag.

Rather than forcing you to code the logic for caching and for invalidating (refreshing) it once the data has changed, HTTP uses two models, expiration and validation, which usually are much simpler to use.

Apart from using Cake\Http\Response::withCache(), you can also use many other methods to fine-tune HTTP cache headers to take advantage of browser or reverse proxy caching.

The Cache Control Header

Cake\Http\Response::withSharable($public, $time = null)

Used under the expiration model, this header contains multiple indicators that can change the way browsers or proxies use the cached content. A Cache-Control header can look like this:

Cache-Control: private, max-age=3600, must-revalidate

Response class helps you set this header with some utility methods that will produce a final valid Cache-Control header. The first is the withSharable() method, which indicates whether a response is to be considered sharable across different users or clients. This method actually controls the public or private part of this header. Setting a response as private indicates that all or part of it is intended for a single user. To take advantage of shared caches, the control directive must be set as public.

The second parameter of this method is used to specify a max-age for the cache, which is the number of seconds after which the response is no longer considered fresh:

public function view()
    // ...
    // Set the Cache-Control as public for 3600 seconds
    $this->response = $this->response->withSharable(true, 3600);

public function my_data()
    // ...
    // Set the Cache-Control as private for 3600 seconds
    $this->response = $this->response->withSharable(false, 3600);

Response exposes separate methods for setting each of the directives in the Cache-Control header.

The Expiration Header


You can set the Expires header to a date and time after which the response is no longer considered fresh. This header can be set using the withExpires() method:

public function view()
    $this->response = $this->response->withExpires('+5 days');

This method also accepts a DateTime instance or any string that can be parsed by the DateTime class.

The Etag Header

Cake\Http\Response::withEtag($tag, $weak = false)

Cache validation in HTTP is often used when content is constantly changing, and asks the application to only generate the response contents if the cache is no longer fresh. Under this model, the client continues to store pages in the cache, but it asks the application every time whether the resource has changed, instead of using it directly. This is commonly used with static resources such as images and other assets.

The withEtag() method (called entity tag) is a string that uniquely identifies the requested resource, as a checksum does for a file, in order to determine whether it matches a cached resource.

To take advantage of this header, you must either call the checkNotModified() method manually or include the Checking HTTP Cache in your controller:

public function index()
    $articles = $this->Articles->find('all')->all();

    // Simple checksum of the article contents.
    // You should use a more efficient implementation
    // in a real world application.
    $checksum = md5(json_encode($articles));

    $response = $this->response->withEtag($checksum);
    if ($response->checkNotModified($this->request)) {
        return $response;

    $this->response = $response;
    // ...


Most proxy users should probably consider using the Last Modified Header instead of Etags for performance and compatibility reasons.

The Last Modified Header


Also, under the HTTP cache validation model, you can set the Last-Modified header to indicate the date and time at which the resource was modified for the last time. Setting this header helps CakePHP tell caching clients whether the response was modified or not based on their cache.

To take advantage of this header, you must either call the checkNotModified() method manually or include the Checking HTTP Cache in your controller:

public function view()
    $article = $this->Articles->find()->first();
    $response = $this->response->withModified($article->modified);
    if ($response->checkNotModified($this->request)) {
        return $response;
    // ...

The Vary Header


In some cases, you might want to serve different content using the same URL. This is often the case if you have a multilingual page or respond with different HTML depending on the browser. Under such circumstances you can use the Vary header:

$response = $this->response->withVary('User-Agent');
$response = $this->response->withVary('Accept-Encoding', 'User-Agent');
$response = $this->response->withVary('Accept-Language');

Sending Not-Modified Responses

Cake\Http\Response::checkNotModified(Request $request)

Compares the cache headers for the request object with the cache header from the response and determines whether it can still be considered fresh. If so, deletes the response content, and sends the 304 Not Modified header:

// In a controller action.
if ($this->response->checkNotModified($this->request)) {
    return $this->response;

Setting Cookies

Cookies can be added to response using either an array or a Cake\Http\Cookie\Cookie object:

use Cake\Http\Cookie\Cookie;
use DateTime;

// Add a cookie
$this->response = $this->response->withCookie(Cookie::create(
    // All keys are optional
        'expires' => new DateTime('+1 year'),
        'path' => '',
        'domain' => '',
        'secure' => false,
        'httponly' => false,
        'samesite' => null // Or one of CookieInterface::SAMESITE_* constants

See the Creating Cookies section for how to use the cookie object. You can use withExpiredCookie() to send an expired cookie in the response. This will make the browser remove its local cookie:

$this->response = $this->response->withExpiredCookie(new Cookie('remember_me'));

Setting Cross Origin Request Headers (CORS)

The cors() method is used to define HTTP Access Control related headers with a fluent interface:

$this->response = $this->response->cors($this->request)
    ->allowMethods(['GET', 'POST'])

CORS related headers will only be applied to the response if the following criteria are met:

  1. The request has an Origin header.

  2. The request’s Origin value matches one of the allowed Origin values.


CakePHP has no built-in CORS middleware because dealing with CORS requests is very application specific. We recommend you build your own CORSMiddleware if you need one and adjust the response object as desired.

Common Mistakes with Immutable Responses

Response objects offer a number of methods that treat responses as immutable objects. Immutable objects help prevent difficult to track accidental side-effects, and reduce mistakes caused by method calls caused by refactoring that change ordering. While they offer a number of benefits, immutable objects can take some getting used to. Any method that starts with with operates on the response in an immutable fashion, and will always return a new instance. Forgetting to retain the modified instance is the most frequent mistake people make when working with immutable objects:

$this->response->withHeader('X-CakePHP', 'yes!');

In the above code, the response will be lacking the X-CakePHP header, as the return value of the withHeader() method was not retained. To correct the above code you would write:

$this->response = $this->response->withHeader('X-CakePHP', 'yes!');