First-class callable syntax in PHP - Mini Project: Build & Apply

Using First-class Callable Syntax in PHP

📖 Scenario: Imagine you are building a simple PHP program that processes a list of numbers by applying a function to each number. You want to use PHP's first-class callable syntax to pass functions easily.

🎯 Goal: You will create a list of numbers, define a function to double a number, use first-class callable syntax to apply this function to each number, and then print the results.

📋 What You'll Learn

Create an array called numbers with the values 1, 2, 3, 4, 5.

Create a function called double that takes one parameter and returns its value multiplied by 2.

Create a variable called doubleFunc that stores the first-class callable syntax for the double function.

Use array_map with doubleFunc to create a new array called doubledNumbers.

Print the doubledNumbers array using print_r.

💡 Why This Matters

🌍 Real World

First-class callable syntax helps you pass functions around easily, making your code cleaner and more flexible when processing data.

💼 Career

Understanding first-class callables is useful for PHP developers working on modern codebases, especially when using functional programming techniques or frameworks.

1

Create the numbers array

Create an array called numbers with the values 1, 2, 3, 4, 5.

PHP

# Create the array called numbers with values 1, 2, 3, 4, 5
# Your code here

Hint

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

2

Define the doubling function

Create a function called double that takes one parameter $n and returns $n * 2.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];

// Define the function double
# Your code here

Hint

Define a function using the function keyword and return the doubled value.

3

Create the first-class callable variable

Create a variable called doubleFunc that stores the first-class callable syntax for the double function using double(...).

PHP

<?php
$numbers = [1, 2, 3, 4, 5];

function double($n) {
    return $n * 2;
}

// Store the first-class callable for double in doubleFunc
# Your code here

Hint

Use double(...) to get the first-class callable and assign it to $doubleFunc.

4

Apply the function and print results

Use array_map with $doubleFunc and $numbers to create a new array called doubledNumbers. Then print doubledNumbers using print_r.

PHP

<?php
$numbers = [1, 2, 3, 4, 5];

function double($n) {
    return $n * 2;
}

$doubleFunc = double(...);

// Use array_map with doubleFunc and numbers to create doubledNumbers
// Print doubledNumbers
# Your code here

Hint

Use array_map($doubleFunc, $numbers) to apply the function to each number.

Practice

1. What does the first-class callable syntax in PHP allow you to do?

easy

A. Automatically convert functions to strings

B. Create new functions dynamically at runtime

C. Prevent functions from being called more than once

D. Treat functions as variables to pass or store them easily

Solution

1. Step 1: Understand the concept of first-class callables

   First-class callable syntax lets you treat functions like variables, so you can pass or store them.

2. Step 2: Compare options with this concept

   Only 'Treat functions as variables to pass or store them easily' correctly describes this ability; others describe unrelated features.

3. Final Answer:

   Treat functions as variables to pass or store them easily -> Option D

4. Quick Check:

   First-class callable = Treat functions as variables [OK]

Hint: Remember: first-class means functions act like variables [OK]

Common Mistakes:

• Confusing callable syntax with dynamic function creation
• Thinking it converts functions to strings
• Assuming it restricts function calls

2. Which of the following is the correct syntax to get a first-class callable for a function named foo in PHP?

easy

A. $callable = foo(...);

B. $callable = foo();

C. $callable = foo\(...);

D. $callable = foo;

Solution

1. Step 1: Recall the first-class callable syntax

   In PHP, to get a callable for a function, you use function_name(...).

2. Step 2: Match the syntax with options

   $callable = foo(...); uses foo(...), which is correct. $callable = foo(); calls the function. foo\(...) uses backslash incorrectly causing syntax error. $callable = foo; treats name as constant.

3. Final Answer:

   $callable = foo(...); -> Option A

4. Quick Check:

   Callable syntax = function_name(...) [OK]

Hint: Use parentheses with ellipsis to get callable: foo(...) [OK]

Common Mistakes:

• Calling the function instead of getting callable
• Omitting parentheses and ellipsis
• Using backslash incorrectly for global functions

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

$fn = strlen(...);
echo $fn('hello');

medium

A. 5

B. hello

C. strlen

D. Error: Call to undefined function

Solution

1. Step 1: Understand the callable assignment

   The variable $fn is assigned the callable for the built-in function strlen using first-class callable syntax.

2. Step 2: Call the callable with argument 'hello'

   Calling $fn('hello') executes strlen('hello'), which returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   strlen('hello') = 5 [OK]

Hint: Callable with strlen(...) returns string length [OK]

Common Mistakes:

• Expecting the function name as output
• Confusing callable with string
• Thinking it causes an error

4. Identify the error in the following PHP code using first-class callable syntax:

function greet($name) {
    return "Hello, $name!";
}

$greet = greet;
echo $greet('World');

medium

A. No error, code runs fine

B. Missing parentheses and ellipsis to get callable

C. Cannot assign function to variable

D. Function greet is not defined

Solution

1. Step 1: Check how callable is assigned

   The code assigns $greet = greet; which is invalid syntax for first-class callable.

2. Step 2: Correct syntax requires parentheses and ellipsis

   To get a callable, it should be $greet = greet(...);. Without this, PHP treats it as a constant or causes error.

3. Final Answer:

   Missing parentheses and ellipsis to get callable -> Option B

4. Quick Check:

   Callable syntax needs (...) [OK]

Hint: Always use (...) to get callable, not just function name [OK]

Common Mistakes:

• Assigning function name without parentheses
• Assuming function name alone is callable
• Ignoring syntax errors

5. Given the following PHP code, what will be the output?

function multiply($a, $b) {
    return $a * $b;
}

$operations = [
    'double' => multiply(...),
    'square' => fn($x) => multiply($x, $x),
];

echo $operations['double'](5, 2) . ', ' . $operations['square'](4);

hard

A. Error: Cannot use multiply(...) here

B. 7, 16

C. 10, 16

D. 10, 8

Solution

1. Step 1: Understand the callable assignments

   The array $operations stores two callables: 'double' uses first-class callable multiply(...), 'square' uses an arrow function calling multiply.

2. Step 2: Evaluate each callable call

   $operations['double'](5, 2) calls multiply(5, 2) = 10.
   $operations['square'](4) calls multiply(4, 4) = 16.

3. Final Answer:

   10, 16 -> Option C

4. Quick Check:

   multiply(5,2)=10 and multiply(4,4)=16 [OK]

Hint: First-class callable works inside arrays like normal functions [OK]

Common Mistakes:

• Confusing arrow function with callable syntax
• Expecting error from multiply(...) usage
• Mixing up multiply arguments