// example form in register page
<form action="post.php">

<input type="hidden" name="action" value="register">

<input type="text" name="email">

<input type="text" name="username">

<input type="password" name="email">


<input type="submit" value="register">

</form>


//post.php file

// check to see what action we are dealing with
if( isset($_POST['action']) && $_POST['action'] == 'register' ) {

 // check to see if all the fields have been posted
 if( isset($_POST['username']) && isset($_POST['email']) && isset($_POST['password']) ) {

   // store into variables for use
   $email = $_POST['email'];
   $username = $_POST['username'];
   $password = $_POST['password'];
   //rest of code...

 }

}