$_GET & $_SET
everyone
The POST method, data is invisible to others.
cross-site scripting
$_REQUEST
Because the data is visible to everyone.
There are seven validation filters. They are FILTER_VALIDATE_EMAIL, FILTER_VALIDATE_BOOLEAN, FILTER_VALIDATE_FLOAT, FILTER_VALIDATE_INT, FILTER_VALIDATE_IP, FILTER_VALIDATE_REGEXP, FILTER_VALIDATE_URL.
FILTER_VALIDATE_EMAIL
strip_tags()