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 only true, never 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

See also PHP Boolean: Assigning True or False to a Variable.

Related : Boolean, Type System, False, Logical Operators, Minus One -1, PHP Natives, Standalone Types, Truthy