Closures as callbacks in PHP - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a closure that prints a message.

PHP

<?php
$message = function() { echo [1]; };
$message();
?>

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

A"Hello, world!"

BHello, world!

Cecho "Hello, world!"

Dprint("Hello, world!")

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the string.

Trying to echo inside the closure argument.

The closure must echo a string, so the string must be quoted.

2fill in blank

medium

Complete the code to pass a closure as a callback to array_map.

PHP

<?php
$numbers = [1, 2, 3];
$squares = array_map([1], $numbers);
print_r($squares);
?>

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

Afunction($n) { echo $n; }

Bfunction($n) { return $n * $n; }

Cfn($n) => $n + $n

Dfunction $n { return $n * $n; }

Attempts:

3 left

💡 Hint

Common Mistakes

Missing parentheses around the parameter.

Using echo instead of return.

The closure must be a function that returns the square of the number.

3fill in blank

hard

Fix the error in the closure that uses a variable from the parent scope.

PHP

<?php
$multiplier = 3;
$multiply = function($n) [1] {
    return $n * $multiplier;
};
echo $multiply(5);
?>

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

Aimport ($multiplier)

Bwith ($multiplier)

Cuse ($multiplier)

Dcapture ($multiplier)

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect keywords like 'with' or 'import'.

Not including the variable in the use clause.

In PHP, to use a variable from the parent scope inside a closure, you must use the use keyword.

4fill in blank

hard

Fill both blanks to create a closure that modifies a variable by reference.

PHP

<?php
$count = 0;
$increment = [1] use ([2]) {
    $count++;
};
$increment();
echo $count;
?>

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

Afunction()

B&$count

C$count

Dfunction(&$count)

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the ampersand to pass by reference.

Trying to pass the variable as a parameter instead of use clause.

The closure is defined as a function with no parameters, but it uses $count by reference with &$count in the use clause.

5fill in blank

hard

Fill all three blanks to create a closure that filters an array using a condition.

PHP

<?php
$words = ['apple', 'bee', 'cat', 'dog'];
$minLength = 3;
$longWords = array_filter($words, function([1]) use ([2]) {
    return strlen([3]) > [2];
});
print_r($longWords);
?>

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

A$word

B$minLength

D$length

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names.

Not passing the external variable with use.

The closure takes $word as parameter, uses $minLength from parent scope, and checks the length of $word.

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