check user php

<?php
//for connecting to the database
mysql_connect("mysql7.000webhost.com","a8839106_enoch","dragon1") or
die(mysql_error());

mysql_select_db("users") or die (mysql_error());
//if the form has been submitted
if (isset($_POST['submit'])) {
 //in case they leave out blank fields
if (!$_POST['username'] | !$_POST['pass'] | !$_POST['email']) {
die ('Please fill in all required fields');
}
//check if username is in use
if (!get_magic_quotes_gpc()) {

$_POST['username'] = addslashes($_POST['username']);  }

$usercheck= $_POST['username'];
$check =mysql_query("SELECT username FROM users WHERE username= '$usercheck'")

or die("Username in use");
//second check
if($check2 !=0) {
die ('Sorry,the username '.$_POST['username'].' is already in use.');
//password encryption using md5
$_POST['pass'] = md5($_POST['pass']);
if (!get_magic_quotes_gpc()) {
$_POST['pass'] = addslashes($_POST['pass']);
$_POST['username'] = addslashes($_POST['username']);
}

//inserting it in the database
$insert = "INSERT INTO users (username, password)
VALUES ('".$_POST['username']."', '".$_POST['pass']."')";
$add_user = mysql_query($insert);

echo "<h1> Welcome! You were successfully
registered! </h1>";

echo '<h1> <a href="login.html"> Go to the signin page </a> </h1>';
?>