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PHP 7+ supports parameter types, return types, and (PHP 7.4+) typed properties. Use them everywhere.

<?php
declare(strict_types=1);    // makes type checks strict — recommended

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

class User {
    public string $name;
    public int $age;
    public ?string $email;        // nullable
}

Without strict_types, PHP coerces — add("5", "10") becomes add(5, 10). With strict_types, it throws TypeError.

Type modifiers:

• ?T — nullable (T or null)
• T|U — union type (PHP 8+)
• iterable, callable, mixed, void, never
• Class names as types: function process(User $u)

Property type rules:

• Properties must be initialized OR be nullable
• private string $x = ""; ✓
• private string $x; errors at construction unless set in __construct

Constructor property promotion (PHP 8+) — declare and assign in one go:

class Point {
    public function __construct(
        public readonly float $x,
        public readonly float $y,
    ) {}
}

Judge0 ships PHP 7.4 — promotion isn't available there, but typed properties are.