How to set a cookie

How to set a cookie on "site1.com"
I only want to use the cookie on "site1.com/file.php"

I call "site1.com/file.php" from <iframe> on site2.com
(<iframe scr=site1.com/file.php></iframe>)

My "file.php" looks like:

<?PHP
setcookie("myCookie", "true", time()+86400,"/");
?>

But I can't set the cookie, - why 😕

If you want to set the cookie for another domain, you'll need to make sure you specifiy that other domain when setting the cookie.