Introduced in PHP 7: scalar types

Code
<?php

$foo 
= function (int $val1float $val2) : string {
    
dump($val1$val2);
    return 
$val1 $val2;
};

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

^ 6.0

^ "10"
Used Versions
PHP 8.0, Laminas MVC 3.2, Symfony 5.2, Laravel 8.28, PHPUnit 9.5, Doctrine ORM 2.8