Declare strict_types directive in PHP

Introduction

The declare(strict_types=1); directive makes PHP check the exact types of values passed to functions. This helps catch mistakes early and keeps your code safer.

When you want to make sure functions only get the exact type of data they expect.

When working on a team project to avoid confusion about what types are allowed.

When you want to find bugs caused by wrong data types quickly.

When writing code that needs to be very reliable and predictable.

When learning PHP and wanting to understand how types work clearly.

Syntax

PHP

<?php
declare(strict_types=1);

// Your PHP code here

This directive must be the very first statement in the PHP file, before any other code or whitespace.

It only affects type checking for function calls in the same file.

Examples

This example uses strict types so add only accepts integers exactly.

PHP

<?php
declare(strict_types=1);

function add(int $a, int $b): int {
    return $a + $b;
}

echo add(5, 10);

Without declare(strict_types=1);, PHP converts "10" to 10 automatically.

PHP

<?php

function add(int $a, int $b): int {
    return $a + $b;
}

// Without strict_types, this works:
echo add(5, "10");

Sample Program

This program multiplies two numbers. Because of strict types, both must be floats exactly. The commented line shows what happens if types are wrong.

PHP

<?php
declare(strict_types=1);

function multiply(float $x, float $y): float {
    return $x * $y;
}

// Correct types
echo multiply(2.5, 4.0) . "\n";

// Incorrect type - this will cause a TypeError
// echo multiply(2, "3");

Important Notes

If you forget to put declare(strict_types=1); at the top, PHP will not check types strictly.

Strict types only affect scalar types like int, float, string, and bool.

Type errors caused by strict types help you find bugs early.

Summary

declare(strict_types=1); makes PHP check exact types for function calls.

Put it at the very top of your PHP file before any code.

It helps catch bugs and makes your code safer and clearer.