Prevento direct access to action

Prevento direct access to action

I'm trying to protect a form action file from direct access: I have
'form.php' and action point to 'form_sql.php'
On form.php I have addedd:
$access = 'allowed';
And on form_sql.php:
if(empty($access)) {
die("direct access not allowed");
}
My trouble is after send form i have direct access not allowed message...
Any help to solve?