Introduced in PHP 7: scalar types

Code
<?php

$foo = function (int $val1, float $val2) : string {
    dump($val1, $val2);
    return $val1 + $val2;
};

dump($foo('4', 6));
Result
^ 4

^ 6.0

^ "10"
Used Versions
PHP 8.3, Laminas MVC 3.2, Symfony 5.2, Laravel 8.28, PHPUnit 9.5, Doctrine ORM 2.8