Description:
------------
There are filter_var() and idn_to_ascii(). 

- filter_var('bob@example.com', FILTER_VALIDATE_EMAIL);
- idn_to_ascii()

But the filter_var('bob@example.com', FILTER_VALIDATE_EMAIL) is not able to check international Domains. So it would be eligible if filter_var() itself can intern e.g. via idn_to_ascii() convert a international Domain bevor check.

This time it is only doable with a workaround - see Below "Test Script"

Test script:
---------------
function validate_email($mail)
{
    if ( ! filter_var($mail, FILTER_VALIDATE_EMAIL) ) {
        $in = explode('@', $mail);
        $in[1] = idn_to_ascii($in[1]);
        return (bool) filter_var(join('@', $in), FILTER_VALIDATE_EMAIL);
    }

    return true;
}

$mail = "foo@müller.de";
var_dump( validate_email($mail) );