Declare strict_types directive in PHP - Mini Project: Build & Apply

Declare strict_types directive

📖 Scenario: You are writing a PHP script that performs simple math operations. To ensure your code treats numbers strictly as integers and avoids unexpected type juggling, you want to enable strict typing.

🎯 Goal: Learn how to declare the strict_types directive in PHP and see how it affects function behavior.

📋 What You'll Learn

Create a PHP file starting with the strict_types declaration

Define a function that accepts an integer parameter

Call the function with an integer and a string representing a number

Observe the difference strict typing makes

💡 Why This Matters

🌍 Real World

Strict typing helps developers write more reliable PHP code by catching type errors early, which is important in large projects and APIs.

💼 Career

Many PHP jobs require understanding strict typing to maintain code quality and avoid bugs caused by unexpected type conversions.

1

Add the strict_types directive

At the very top of your PHP file, write declare(strict_types=1); to enable strict typing.

PHP

<?php
declare(strict_types=1);
# Your code here

Need a hint?

The declare(strict_types=1); directive must be the first statement after the opening <?php tag.

2

Define a function with an integer parameter

Define a function called double that takes one parameter int $number and returns $number * 2.

PHP

<?php
declare(strict_types=1);

function double(int $number): int {
    # Your code here
}

Need a hint?

Use int $number to enforce the parameter type and : int to specify the return type.

3

Call the function with an integer and a string

Call the double function twice: once with the integer 5 and once with the string '5'. Assign the results to variables $result1 and $result2 respectively.

PHP

<?php
declare(strict_types=1);

function double(int $number): int {
    return $number * 2;
}

$result1 = double(5);
# Your code here

Need a hint?

Call the function exactly as double(5) and double('5') and assign to the specified variables.

4

Print the results

Print $result1 and $result2 each on a new line using echo. This will show how strict typing affects the second call.

PHP

<?php
declare(strict_types=1);

function double(int $number): int {
    return $number * 2;
}

$result1 = double(5);
$result2 = double('5');

echo $result1 . "\n";
# Your code here

Need a hint?

Because strict typing is enabled, calling double('5') will cause a TypeError and the script will stop before printing the second result.