False

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

<?php

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

?>

Documentation

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

Related : Boolean, Type System, True, Falsy, Truthy, Underscore, zero, Logical Operators, file_get_contents(), PHP Natives, one, Short Ternary Operator, Autovivification, Standalone Types