PHP query not working using form

This is my query for inserting:

<?php
    require_once "config.php";
    if(isset($_POST['submit'])){

    $email = mysql_escape_string($_POST['txtemail']);

    $str = "insert into subscribe(email,datetime) values ('$email',current_timestamp)";
    $result = mysql_query($str);

    print '<script type="text/javascript">';
            print 'alert("Thank you for subscribing with us. We will be in touch with you very soon.")';
            print '</script>';  


}

this is code above is perfectly working, but when i put a value on the form action to redirect to another page like this, the query is not inserting.

<form action="/?sender=nopopup" method="POST">

I tried using header(location: 'url'), but its not working also. Any help will be appreciated, thank you.