Arrow functions (short closures) in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an arrow function that returns the square of a number.

PHP

$square = fn($x) => [1];
echo $square(4);

Drag options to blanks, or click blank then click option'

A$x + $x

B2 * $x

C$x - 2

D$x * $x

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication.

Forgetting to use the variable inside the function.

The arrow function returns the square by multiplying the input by itself.

2fill in blank

medium

Complete the code to create an arrow function that adds 10 to a number.

PHP

$addTen = fn($num) => [1];
echo $addTen(5);

Drag options to blanks, or click blank then click option'

A$num + 10

B$num * 10

C10 - $num

D$num - 10

Attempts:

3 left

💡 Hint

Common Mistakes

Subtracting instead of adding.

Multiplying instead of adding.

The arrow function adds 10 to the input number.

3fill in blank

hard

Fix the error in the arrow function that should return the length of a string.

PHP

$length = fn($str) => [1];
echo $length('hello');

Drag options to blanks, or click blank then click option'

Acount($str)

Blength($str)

Cstrlen($str)

Dsize($str)

Attempts:

3 left

💡 Hint

Common Mistakes

Using length() which is not a PHP function.

Using count() which is for arrays.

The correct PHP function to get string length is strlen().

4fill in blank

hard

Fill both blanks to create an arrow function that filters an array to keep only numbers greater than 5.

PHP

$filter = fn($arr) => array_filter($arr, fn($n) => $n [1] [2]);
print_r($filter([3, 7, 2, 9]));

Drag options to blanks, or click blank then click option'

A>

B5

C<

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of >.

Using wrong threshold number.

The arrow function uses > 5 to keep numbers greater than 5.

5fill in blank

hard

Fill all three blanks to create an arrow function that maps an array to its elements doubled if they are even.

PHP

$doubleEven = fn($arr) => array_map(fn($x) => [1] ? [2] : [3], $arr);
print_r($doubleEven([1, 2, 3, 4]));

Drag options to blanks, or click blank then click option'

A$x % 2 === 0

B$x * 2

C$x

D$x + 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition instead of multiplication for doubling.

Not checking for even numbers correctly.

The arrow function checks if a number is even, doubles it if true, otherwise returns it unchanged.

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