PHP Closures in Simple English

Learn what are PHP closures in really simple English that everyone understand. This article covers everything including use cases too.

Closure is a PHP function without a name. If you see any function in PHP without a name then it is a closure. Closures can be

Stored in a variable
Passed as a argument in another function.
Set as a return value of a function.

Storing a closure in a variable

You can store a closure in a variable since PHP closures does not have a name.

$greet = function($name){
   return "Hello, $name";
}

echo $greet('bob');

Did you notice that using a variable as a function. Normally variables store integers, strings, arrays, etc. not functions. With closures you can store a function in a variable.

Passing a closure as a Argument

You can pass a closure as a Argument in another function. Mostly called as callback function. You may have used this feature very often.

Task: create a function that accept two parameters as name and callback function. Write a callback function to add greetings to your name. ex: hello, bob. If callback is not provided just return the name.

function sayMyName($name, $callback=null){
  if(is_callable($callback)){
    return $callback($name);
  }
  return $name;
}

echo sayMyName("bob", function($name){
  echo "hello, $name";
});

Task: create a callback function or a closure to process numbers of a integer array and return its values doubled. Use closure as the argument of the function.

function processArray(array $array, callable $callback)
{
  $result = [];
  foreach($array as $item)
  {
    $result[] = $callback($item);
  }

  return $result;
}

$numberAry = [1,2,3,4,5,6];
$doubledArray = processArray($numberAry, function($number)
{
  return $number * $number;
});

print_r($doubledArray);

Set closure as the return value of a function

Here define a closure to accept one parameter called $name and add add greeting message for the return value of a function.

function addGreetings()
{
  return function($name){
    return "hello, $name";
  };
}

$greetings = addGreetings();
echo $greetings('bob');

Make sure to add semi colon to returning function or closure to end the statement. Normally sem colon (;) is not added at the end of a function. Here function is used as a statement.

Questions

Task 1

Use a closure with array_map function. Instead of using a normal PHP function for callback to create a new array use a closure. Result of the element of the array is the square of the corresponding element in the original array?

Solution:

$numAry = [2,4,6,8,10];
print_r (array_map(function($number){
  return $number * $number;
}, $numAry));

Task 2

Create a immediately invoked closure to add two numbers. Such as inputting 3 and 4 to get 7 as the result.

Step 1: create a closure to accept two numbers and return the sum

function($x, $y){
  return $x+$y;
}

Step 3: In PHP you can immediately invoke this function by wrapping the function with parenthesis “()” and immediately opening another set of parenthesis right after that.

echo (function($x, $y){
  return $x+$y;
})(4,3);

Task 3

Define a middleware closure in PHP to perform actions both before and after processing a request.

solution:

$middleware = function($request, $next)
{
  //perform action before the request
  $request = $request." -pre request";

  //Pass the request to next handle like controller, middleware, etc.
  $request = $next($request);

  //Perform action after the request like before sending response to user.
  return $request = $request." - post request";
};

//defining the next handler
$nextHandler = function($request){
  return $request." - next handler data";
};

echo $middleware("initial request", $nextHandler); 
//output - initial request -pre request - next handler data - post request

Task 4

Implement a basic service container in PHP using closures and bind and resolve services dynamically. In other word use closures for dependency injection.

Solution – This is for those who didn’t have a clue about service containers and dependency injection practices.

Service container stores list of services or classes with business logic to do something.
Service container should be able to deliver initialized object of the defined service or class when requested.
Place where the service container is used to create a service like controller does not know how the service is created and its dependencies.
Services are created or objects are initialized only when needed. Creating all the services without a need will max out resource usage.
This is called dependency injection. Where required services or initialized object classes are injected in the required class like controller class by a service container or dependency injector only when needed.

step 1: Create a service container ( just a normal PHP class. ) to hold all the service classes. It should need a array variable to hold service classes, method to add a service to container and lastly a method to create the service (initialize the class or object).

class ServiceContainer{

  //stores all the service class names as the key.
 //stores how to initialize logic in a closure as the value
  protected $services = [];

  /**
   * Register services with the container.
   * $name - name of the service
   * $closure - define how to create the service
   */
  public function bind(string $serviceName, Closure $closure){
    $this->services[$serviceName] = $closure;
  }

  /**
   * get the requested closure function and execute it.
   */
  public function make(string $serviceName){
    //() used at the end to call the return closure and pass the 
    //$this variable to it. So it can uses any variable or resources 
    //used in the service container if needed.
    return $this->services[$serviceName]($this);
  }
}

Step 2: create a sample service class. Here there is nothing unordinary. Just a simple PHP class with a method.

class ExampleService{
  public function firstMethod(){
    echo "Hello, I am the first method of the example service";
  }
}

step 3: Initialize the service container and add above example service class to it.

//Intializing the service container class.
$serviceContaier = new ServiceContainer();

//Binding our example service class to service container.
$serviceContaier->bind('exampleService', function(){
  //Here define how to create an instance of the service class. Which is isolated 
  //from the place where it uses this instance.
  return new ExampleService();
});

step 4: create a service object using the service container and use created service object.

//Get a service class object from the service container.
$exampleService = $serviceContaier->make('exampleService');

//Using the initialized or created service object by service container.
echo $exampleService->firstMethod();

Below shows full example.

<?php

class ServiceContainer{

  //stores all the service class names as the key.
 //stores how to initialize logic in a closure as the value
  protected $services = [];

  /**
   * Register services with the container.
   * $name - name of the service
   * $closure - define how to create the service
   */
  public function bind(string $serviceName, Closure $closure){
    $this->services[$serviceName] = $closure;
  }

  /**
   * get the requested closure function and execute it.
   */
  public function make(string $serviceName){
     //() used at the end to call the return closure and pass the 
    //$this variable to it. So it can uses any variable or resources 
    //used in the service container if needed.
    return $this->services[$serviceName]($this);
  }
}

//Example service class.
class ExampleService{
  public function firstMethod(){
    echo "Hello, I am the first method of the example service";
  }
}

//Intializing the service container class.
$serviceContaier = new ServiceContainer();

//Binding our example service class to service container.
$serviceContaier->bind('exampleService', function(){
  return new ExampleService();
});

//Get a service class object from the service container.
$exampleService = $serviceContaier->make('exampleService');

//Using the initialized or created service object by service container.
echo $exampleService->firstMethod();