PHP declare(strict_types=1) trap

index.php

<?php
// No strict type here...

require_once __DIR__ . '/strict.php';
function foo(/* int */ $input) { // ..so adding type definition is safe, isn't it?
    printf("param=%d\n", $input);
}

strict.php

<?php
declare(strict_types=1);

foo(5);
foo('7'); // but it fails, because it is called from file with declare(strict_types=1);