Vyřešeno:

Do 2.php stačilo dát funkci:

function pathToURL() {
	$path = str_replace("\\", "/", realpath(dirname(__FILE__)));
	$pos = strpos($path, $_SERVER['DOCUMENT_ROOT']);
	if ($pos === false) return false;
	return (isset($_SERVER['HTTPS']) ? 'https' : 'http').'://'.$_SERVER['SERVER_NAME'].substr($path, strlen($_SERVER['DOCUMENT_ROOT']));
}

a pak ji v 2.php zavolat:

".pathToURL().'/images/3.png'."

1.php se nezměnilo