add.php

<?php

function add(int $a, int $b): int {
    myLog($a + $b);
    return $a + $b;
}

logger.php

<?php
declare(strict_types=1);

function myLog(string $message): string {
  return $message;
}

main.php

<?php

require 'add.php';
require 'logger.php';

$result = add(1, 3);
echo $result;

output.txt

$ php main.php
4

$ hhvm -vEval.EnableHipHopSyntax=true main.php
Catchable fatal error: Argument 1 passed to myLog() must be an instance of string, int given in /home/vagrant/separate_files_mixed/logger.php on line 6