Weak comparisons

Key principles

Follow these fundamental rules when writing compatible code:

Analyze variable types: Always understand what types and values a variable can hold.

Use strict comparisons: Prefer === and !== over == and != to avoid implicit type coercion.

Declare types explicitly: Always use type declarations for function/method parameters and return types.

Validate with strict checks: Use strict type checks in conditional statements to prevent unexpected behavior.behavior (e.g., in_array with true as third parameter).

Use PHP constants: Always use predefined PHP constants instead of numbers (e.g., UPLOAD_ERR_OK instead of 0).

Enable strict types: Add declare(strict_types=1); at the top of your PHP files when possible.

Strings

Use empty() instead of comparing with empty string.

if ($a == '') {
    // ...
}

if (empty($a)) {
    // ...
}

if ($a != '') {
    // ...
}

if (!empty($a)) {
    // ...
}

Checkbox values

In PHP 8, comparing numeric strings with numbers produces different results.

if ($a == 0) {
    // ...
} elseif ($a == 1) {
    // ...
} else {
    // ...
}

if (strval($a) === '0') {
    // ...
} elseif (strval($a) === '1') {
    // ...
} else {
    // ...
}