Arrow functions (short closures) in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an arrow function in PHP?

An arrow function is a short way to write anonymous functions using the fn keyword. It automatically captures variables from the parent scope without needing the 'use' keyword.

beginner

How do arrow functions capture variables from the parent scope?

Arrow functions automatically capture variables from the parent scope by value, so you don't need to specify them with 'use'.

beginner

Rewrite this anonymous function as an arrow function:

$double = function($x) { return $x * 2; };

$double = fn($x) => $x * 2;

intermediate

Can arrow functions contain multiple statements in PHP?

No, arrow functions can only contain a single expression. For multiple statements, use a regular anonymous function.

intermediate

What is the difference between arrow functions and regular anonymous functions in PHP?

Arrow functions use the fn keyword, have implicit returns, and automatically capture variables by value. Regular anonymous functions use the function keyword and require 'use' to access variables from the parent scope.

Which keyword is used to declare an arrow function in PHP?

A fn

B function

C arrow

D lambda

Arrow functions in PHP are declared using the 'fn' keyword.

How do arrow functions capture variables from the parent scope?

A You must specify variables with 'use'

B Automatically by reference

C Automatically by value

D They cannot access parent scope variables

Arrow functions automatically capture variables from the parent scope by value.

Can arrow functions contain multiple statements in PHP?

A Yes, with curly braces

B Yes, if you use 'return'

C Only if declared inside a class

D No, only a single expression

Arrow functions can only contain a single expression; multiple statements require regular anonymous functions.

What does this arrow function return?

$add = fn($a, $b) => $a + $b;
echo $add(3, 4);

A 7

B 34

C Error

D null

The arrow function adds 3 and 4, returning 7.

Which of the following is true about arrow functions compared to regular anonymous functions?

A Arrow functions require 'use' to access variables

B Arrow functions have implicit return

C Arrow functions can have multiple statements

D Arrow functions cannot access parent scope variables

Arrow functions have implicit return and automatically capture variables by value.

Explain how arrow functions differ from regular anonymous functions in PHP.

Think about syntax and how variables are accessed.

Write a simple arrow function in PHP that multiplies a number by 5.

Use fn(parameter) => expression;

Practice

1. What is a key feature of PHP arrow functions (short closures)?

easy

A. They automatically return the value of the expression without needing the return keyword.

B. They require the use of the 'function' keyword every time.

C. They cannot access variables from the parent scope.

D. They must always have multiple statements inside the function body.

Solution

1. Step 1: Understand arrow function syntax

   Arrow functions in PHP use the 'fn' keyword and automatically return the expression value without needing 'return'.

2. Step 2: Compare features

   Unlike regular closures, arrow functions can access variables from the parent scope without 'use'.

3. Final Answer:

   They automatically return the value of the expression without needing the return keyword. -> Option A

4. Quick Check:

   Arrow functions auto-return expression [OK]

Hint: Arrow functions auto-return expressions without 'return' keyword [OK]

Common Mistakes:

• Thinking arrow functions need 'return' keyword
• Believing arrow functions can't access outside variables
• Confusing arrow functions with regular anonymous functions

2. Which of the following is the correct syntax for a PHP arrow function that doubles a number?

easy

A. $double = function($x) { $x * 2; };

B. $double = fn($x) => $x * 2;

C. $double = fn $x => return $x * 2;

D. $double = fn($x) { return $x * 2; };

Solution

1. Step 1: Identify correct arrow function syntax

   Arrow functions use 'fn' keyword, parentheses for parameters, and '=>' followed by expression without braces.

2. Step 2: Check each option

   $double = fn($x) => $x * 2; matches correct syntax: fn($x) => $x * 2;. The other options use regular function syntax, braces incorrectly, or lack parentheses and misuse 'return'.

3. Final Answer:

   $double = fn($x) => $x * 2; -> Option B

4. Quick Check:

   Arrow function syntax = fn(params) => expression [OK]

Hint: Arrow functions use 'fn(params) => expression;' syntax [OK]

Common Mistakes:

• Using braces {} with arrow functions
• Adding 'return' keyword inside arrow functions
• Omitting parentheses around parameters

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

$factor = 3;
$multiply = fn($x) => $x * $factor;
echo $multiply(5);

medium

A. 0

B. Error: Undefined variable

C. 5

D. 15

Solution

1. Step 1: Understand variable scope in arrow functions

   Arrow functions automatically capture variables from the parent scope, so $factor = 3 is accessible inside the function.

2. Step 2: Calculate the output

   Calling $multiply(5) computes 5 * 3 = 15, which is printed.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Arrow functions capture outside variables automatically [OK]

Hint: Arrow functions capture outside variables automatically [OK]

Common Mistakes:

• Expecting 'use' keyword to access outside variables
• Thinking $factor is undefined inside arrow function
• Confusing arrow functions with regular closures

4. Identify the error in this PHP code using an arrow function:

$a = 10;
$fn = fn($x) => { return $x + $a; };
echo $fn(5);

medium

A. Arrow functions cannot use variables from outside scope.

B. Missing 'use' keyword to access $a inside the function.

C. Arrow functions cannot have braces {} around the expression.

D. Arrow functions must have a semicolon after the expression.

Solution

1. Step 1: Check arrow function syntax rules

   Arrow functions use 'fn' with '=>' followed by a single expression without braces or 'return'.

2. Step 2: Identify the error in the code

   The code uses braces {} and 'return' inside an arrow function, which is invalid syntax.

3. Final Answer:

   Arrow functions cannot have braces {} around the expression. -> Option C

4. Quick Check:

   Arrow functions = no braces, no return [OK]

Hint: Arrow functions do not use braces or 'return' keyword [OK]

Common Mistakes:

• Adding braces {} around arrow function body
• Using 'return' inside arrow functions
• Thinking 'use' is needed for outside variables in arrow functions

5. Given this array of numbers, which arrow function code correctly filters out all numbers less than 5 and then doubles the remaining numbers?

$numbers = [2, 4, 5, 7, 1];
// Fill in the blanks:
$filtered = array_filter($numbers, fn($n) => ________);
$doubled = array_map(fn($n) => ________, $filtered);
print_r($doubled);

hard

A. $n > 4; $n * 2

B. $n > 5; $n + 2

C. $n > 5; $n * 2

D. $n >= 4; $n * 2

Solution

1. Step 1: Determine filter condition

   We want to keep numbers 5 or greater. Since 5 is included, condition is $n > 4 (which means 5 and above).

2. Step 2: Determine map operation

   We want to double the filtered numbers, so multiply by 2: $n * 2.

3. Step 3: Check options

   $n > 4; $n * 2 uses $n > 4 for filter and $n * 2 for map, matching requirements exactly.

4. Final Answer:

   $n > 4; $n * 2 -> Option A

5. Quick Check:

   Filter >=5 = $n > 4, double = $n * 2 [OK]

Hint: Filter with $n > 4 to include 5, then multiply by 2 [OK]

Common Mistakes:

• Using wrong comparison operator in filter
• Adding instead of multiplying in map
• Including numbers less than 5 by mistake