I'm also in a "trial and error" for that, but this answer from Google Chrome Labs' Github helped me a little. I defined it into my main file and it worked - well, for only one third-party domain. Still making tests, but I'm eager to update this answer with a better solution :)
EDIT: I'm using PHP 7.4 now, and this syntax is working good (Sept 2020):
$cookie_options = array(
'expires' => time() + 60*60*24*30,
'path' => '/',
'domain' => '.domain.com', // leading dot for compatibility or use subdomain
'secure' => true, // or false
'httponly' => false, // or false
'samesite' => 'None' // None || Lax || Strict
);
setcookie('cors-cookie', 'my-site-cookie', $cookie_options);
If you have PHP 7.2 or lower (as Robert's answered below):
setcookie('key', 'value', time()+(7*24*3600), "/; SameSite=None; Secure");
If your host is already updated to PHP 7.3, you can use (thanks to Mahn's comment):
setcookie('cookieName', 'cookieValue', [
'expires' => time()+(7*24*3600,
'path' => '/',
'domain' => 'domain.com',
'samesite' => 'None',
'secure' => true,
'httponly' => true
]);
Another thing you can try to check the cookies, is to enable the flag below, which—in their own words—"will add console warning messages for every single cookie potentially affected by this change":
chrome://flags/#cookie-deprecation-messages
See the whole code at: https://github.com/GoogleChromeLabs/samesite-examples/blob/master/php.md, they have the code for same-site-cookies too.