PHP sign up code - php

Question

I am creating my first sign in/register function to my web site by following a online tutorial. Every thing seems to be working good , My problem is in the tutorial the php if ($_SERVER['REQUEST_METHOD'] == 'POST') is set in the index page which checks if all the fields and then inserts them into the DB . But for me this not seem to work. But if I put the code onto the page where the form action redirects after it works fine. Is this the right way to do it. I wouldn't like to think so because I would like to check all the variable before we move on.
So if someone would like to educate me on this would be great.
Here is my php code still not fully finished but i wanted to clear this up first.
This is used by include method
<?php
//setup some variables/arrays
$action = array();
$action['result'] = null;
//check if the form has been submitted
if ($_SERVER['REQUEST_METHOD'] == 'POST'){
$message = "wrong answer";
echo "<script type='text/javascript'>alert('$message');</script>";
$firstName = mysqli_real_escape_string($link,$_POST['firstName']);
$lastName = mysqli_real_escape_string($link,$_POST['lastName']);
$password = mysqli_real_escape_string($link,$_POST['sign-up-password']);
$confirmPassword = mysqli_real_escape_string($link,$_POST['password-confirm']);
$email = mysqli_real_escape_string($link,$_POST['email2']);
//quick/simple validation
if(empty($firstName)){ $action['result'] = 'error';}
if(empty($lastName)){ $action['result'] = 'error';}
if(empty($password)){ $action['result'] = 'error';}
if(empty($email)){ $action['result'] = 'error';}
if($password != $confirmPassword){ $action['result'] = 'error';}
if($action['result'] != 'error'){
$add = mysqli_query($link,"INSERT INTO `users` VALUES(NULL,'$firstName','$lastName','$password','$email',0)");
if($add){
//the user was added to the database
//get the new user id
$userid = mysqli_insert_id($link);
//create a random key
$key = $firstName . $email . date('mY');
$key = md5($key);
//add confirm row
$confirm = mysqli_query($link,"INSERT INTO `confirm` VALUES(NULL,'$userid','$key','$email')");
if($confirm){
//let's send the email
}
}else{
$action['result'] = 'error';
array_push($text,'User could not be added to the database. Reason: ' . mysql_error());
}
}else{
}
}
?>