What is the difference between Type Juggling and declare(strict_types=1) in PHP?

Short answer

Type Juggling automatically converts types implicitly, while strict_types requires exact types for parameters and return values and throws TypeError on a mismatch.

Explanation

By default in PHP, if a function expects an int and you pass the string '42', the language converts it implicitly. When declare(strict_types=1) is placed on the first line of a file, implicit conversions are disabled for calls made from that file, and the engine immediately throws TypeError. This prevents hidden errors and improves code safety and stability.

Code example

<?php
declare(strict_types=1);

function add(int $a, int $b): int {
    return $a + $b;
}
add(5, 10); // True
// add("5", 10); // Throws Fatal TypeError

Common mistakes

• Believing that declare(strict_types=1) affects the whole project when placed in one file; it applies only to the file where it is written.

Follow-up questions

• What is the difference between the == (Equal) and === (Identical) comparison operators in PHP 8+?

Sources

• PHP docs — Type declarations