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.

Practice

1. What is a closure in PHP when used as a callback?

easy

A. A special variable type for storing data

B. A named function that cannot access outside variables

C. An anonymous function that can be passed as an argument

D. A PHP keyword for ending loops

Solution

1. Step 1: Understand what a closure is

   A closure is an anonymous function, meaning it has no name and can be created on the fly.

2. Step 2: Recognize closures as callbacks

   Closures can be passed as arguments to other functions to customize behavior, making them useful as callbacks.

3. Final Answer:

   An anonymous function that can be passed as an argument -> Option C

4. Quick Check:

   Closure = anonymous function callback [OK]

Hint: Closures are nameless functions passed as arguments [OK]

Common Mistakes:

• Thinking closures must have names
• Confusing closures with variables
• Believing closures cannot be passed as arguments

2. Which of the following is the correct syntax to define a closure as a callback in PHP?

easy

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

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

C. array_map(function => $x * 2, [1, 2, 3]);

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

Solution

1. Step 1: Check closure syntax

   The correct syntax for an anonymous function uses the keyword function followed by parentheses with parameters, then braces with the function body.

2. Step 2: Validate the return statement

   The closure must explicitly return the value to be used by array_map. array_map(function($x) { return $x * 2; }, [1, 2, 3]); correctly returns $x * 2.

3. Final Answer:

   array_map(function($x) { return $x * 2; }, [1, 2, 3]); -> Option D

4. Quick Check:

   Correct closure syntax = array_map(function($x) { return $x * 2; }, [1, 2, 3]); [OK]

Hint: Use function($param) { return value; } for closures [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Missing return keyword inside closure
• Using arrow (=>) incorrectly in closure

3. What will be the output of the following PHP code?

$numbers = [1, 2, 3];
$result = array_map(function($n) { return $n + 5; }, $numbers);
print_r($result);

medium

A. Array (1, 2, 3, 5)

B. [6, 7, 8]

C. [5, 6, 7]

D. [1, 2, 3]

Solution

1. Step 1: Understand array_map with closure

   The closure adds 5 to each element of the array $numbers.

2. Step 2: Calculate each element

   1+5=6, 2+5=7, 3+5=8, so the resulting array is [6, 7, 8].

3. Final Answer:

   [6, 7, 8] -> Option B

4. Quick Check:

   Each element + 5 = [6,7,8] [OK]

Hint: array_map applies closure to each element [OK]

Common Mistakes:

• Confusing original array with result
• Forgetting closure return value
• Misreading array_map behavior

4. Identify the error in this PHP code using a closure as a callback:

$multiplier = 3;
$numbers = [1, 2, 3];
$result = array_map(function($n) { return $n * $multiplier; }, $numbers);
print_r($result);

medium

A. The variable $multiplier is not accessible inside the closure

B. array_map cannot accept closures as callbacks

C. Missing semicolon after array_map call

D. The closure must be named to access external variables

Solution

1. Step 1: Check variable scope inside closure

   Variables outside a closure are not accessible inside unless passed with use.

2. Step 2: Identify missing use keyword

   The closure needs use ($multiplier) to access the external variable.

3. Final Answer:

   The variable $multiplier is not accessible inside the closure -> Option A

4. Quick Check:

   External vars need use() in closures [OK]

Hint: Use 'use' to access outside variables in closures [OK]

Common Mistakes:

• Assuming closures access all outside variables automatically
• Thinking closures must be named to access variables
• Believing array_map rejects closures

5. How can you modify this PHP code to correctly multiply each number by an external variable using a closure as a callback?

$factor = 4;
$nums = [2, 4, 6];
$results = array_map(function($num) { return $num * $factor; }, $nums);
print_r($results);

hard

A. Add use ($factor) after the function parentheses

B. Declare $factor as global inside the closure

C. Pass $factor as a second argument to array_map

D. Replace closure with a named function

Solution

1. Step 1: Understand variable scope in closures

   To access an external variable inside a closure, you must explicitly import it using use.

2. Step 2: Apply use ($factor) syntax

   Modify the closure to function($num) use ($factor) { return $num * $factor; } to access $factor.

3. Final Answer:

   Add use ($factor) after the function parentheses -> Option A

4. Quick Check:

   use() imports external vars into closures [OK]

Hint: Use 'use' keyword to bring external variables inside closures [OK]

Common Mistakes:

• Trying to access external variables without use()
• Using global keyword inside closures incorrectly
• Passing extra arguments to array_map