True

true has two usages, as PHP keyword : the opposite of false, as a boolean value and a special type for functions that may return a boolean, but not false.

true as a type was introduced in PHP 8.2.

<?php

function foo($a) : true|A {
    if ($a == 1) {
        return true;
    } else {
        return new A();
    }
}

?>

Documentation

Related : Boolean, Type System, False