Closures as callbacks in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a closure in PHP?

A closure is an anonymous function that can capture variables from its surrounding scope and be used as a callback or stored in a variable.

beginner

How do you define a closure to use as a callback in PHP?

You define an anonymous function using the function keyword without a name, optionally capturing variables with use, and pass it as an argument where a callback is expected.

intermediate

What does the use keyword do in a PHP closure?

The use keyword allows a closure to access variables from the parent scope by copying them into the closure's scope.

intermediate

Why are closures useful as callbacks?

Closures let you write small, inline functions that can access variables from outside their scope, making callbacks flexible and easy to customize without defining a full named function.

beginner

Example: What will this PHP code output?

$greet = function($name) { return "Hello, $name!"; };
echo $greet('Alice');

It will output: Hello, Alice! because the closure takes the argument $name and returns a greeting string.

What keyword is used to create an anonymous function (closure) in PHP?

A function

B closure

C lambda

D callback

In PHP, the function keyword is used to create anonymous functions, which are closures.

How do you access a variable from the parent scope inside a closure?

A By passing it as a parameter

B Using the <code>use</code> keyword

C Using <code>global</code>

D You cannot access it

The use keyword imports variables from the parent scope into the closure.

Which of these is a valid way to pass a closure as a callback in PHP?

A array_map(function($x) { return $x * 2; }, [1, 2, 3])

B array_map('function($x) { return $x * 2; }', [1, 2, 3])

C array_map(closure($x) { return $x * 2; }, [1, 2, 3])

D array_map(callback($x) { return $x * 2; }, [1, 2, 3])

You can pass an anonymous function directly as a callback, like in option D.

What will this code output?

$factor = 3;
$multiply = function($n) use ($factor) { return $n * $factor; };
echo $multiply(5);

A 3

B 5

C 15

D Error

The closure uses the $factor variable from the parent scope and multiplies 5 by 3, outputting 15.

Why might you prefer a closure over a named function for a callback?

A Closures always run asynchronously

B Closures cannot access outside variables

C Named functions cannot be used as callbacks

D Closures are faster to write and can capture variables from outside scope

Closures are convenient for quick, inline callbacks and can capture variables from their surrounding scope.

Explain how closures work as callbacks in PHP and how they can access variables from outside their scope.

Think about how you write a small function inside another function and still use outside data.

Describe a simple example of using a closure as a callback with array_map in PHP.

Imagine you want to double every number in a list using a small function.