Arrow functions (short closures) in PHP - Mini Project: Build & Apply

Arrow functions (short closures) in PHP

📖 Scenario: You are working on a small PHP script that processes a list of product prices. You want to apply a discount to each price and then display the discounted prices.

🎯 Goal: Build a PHP script that uses arrow functions (short closures) to apply a discount to each product price and then prints the discounted prices.

📋 What You'll Learn

Create an array called prices with the exact values: 100, 200, 300

Create a variable called discount and set it to 0.1 (which means 10%)

Use an arrow function inside array_map to calculate discounted prices

Print the resulting discounted prices array

💡 Why This Matters

🌍 Real World

Applying discounts or taxes to product prices is common in online stores and billing systems.

💼 Career

Understanding arrow functions helps write concise and modern PHP code, which is valuable for web developers.

1

Create the prices array

Create an array called prices with the exact values 100, 200, and 300.

PHP

<?php

// Create the array called prices with values 100, 200, 300
// Your code here

Hint

Use square brackets [] to create an array in PHP.

2

Set the discount rate

Create a variable called discount and set it to 0.1 to represent a 10% discount.

PHP

<?php

$prices = [100, 200, 300];

// Create a variable called discount and set it to 0.1
// Your code here

Hint

Use the = sign to assign the value 0.1 to the variable discount.

3

Calculate discounted prices using an arrow function

Use array_map with an arrow function to create a new array called discountedPrices. The arrow function should take each price and return the price after subtracting the discount (price minus price times discount).

PHP

<?php

$prices = [100, 200, 300];
$discount = 0.1;

// Use array_map with an arrow function to create discountedPrices
// Your code here

Hint

Use fn($price) => $price - $price * $discount as the arrow function inside array_map.

4

Print the discounted prices

Print the discountedPrices array using print_r to display the discounted prices.

PHP

 $price - $price * $discount, $prices);

// Print the discountedPrices array
// Your code here

Hint

Use print_r($discountedPrices); to print the array in a readable format.

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