Description:
------------
The function return type works for int and bool, but not for integer or boolean.

Test script:
---------------
<?php
// This function works fine.
function foo(): int {
    return 2;
}

// This function does not work because of integer instead of int.
function bar(): integer {
    return 3;
}

// This function works fine.
function baz(): bool {
    return true;
}

// This function does not work because of boolean instead of bool.
function foobar(): boolean {
    return false;
}

echo foo(); // Works fine.
echo bar(); // Fatal error: Uncaught TypeError: Return value of bar() must be an
            //     instance of integer, integer returned in
            //     /var/www/html/index.php on line 7 in
            //     /var/www/html/index.php:7 Stack trace: #0
            //     /var/www/html/index.php(19): bar() #1 {main} thrown in
            //     /var/www/html/index.php on line 7
echo baz(); // Works fine.
echo foobar(); // Fatal error: Uncaught TypeError: Return value of foobar() must
               //     be an instance of boolean, boolean returned in
               //     /var/www/html/index.php on line 15 in 
               //     /var/www/html/index.php:15 Stack trace: #0
               //     /var/www/html/index.php(21): foobar() #1 {main} thrown in
               //     /var/www/html/index.php on line 15