Checking if username exists POSTBACK - php

Question

I want to check using the POSTBACK method i the user exists in mysql table. I studying mysql and i understand it will be removed soon but I cant change at the moment. I want the alert to pop up next to the username text box if it already exists.At the moment it isnt working. I have a similar code for password and password confirmation but i think this differs since i need a query. This is what i have:
<?php
$passErr = $pass1Err = "";
$passw = $passw1 = "";
$userErr="";
if ($_SERVER["REQUEST_METHOD"] == "POST") {
if (empty($_POST["password"])) {
$passErr = "Password is required";
}
if (empty($_POST["passconfirm"])) {
$pass1Err = "Password confirmation is required";
}
if ($_POST['password']!= $_POST['passconfirm'])
{
$passErr = "Passwords must be the same";
$pass1Err = "Passwords must be the same";
}
}
else {
if (isset($_REQUEST["submit"]))
{
if (isset($_POST["submit"]))
{
$firstname = mysql_real_escape_string($_POST["gname"]);
$middlename = mysql_real_escape_string($_POST["mname"]);
$lastname = mysql_real_escape_string($_POST["surname"]);
$user = mysql_real_escape_string($_POST["username"]);
$addy = mysql_real_escape_string($_POST["address"]);
$post = mysql_real_escape_string($_POST["postcode"]);
$sta = mysql_real_escape_string($_POST["state"]);
$telephone = mysql_real_escape_string($_POST["tel"]);
$pass = mysql_real_escape_string($_POST["password"]);
$systemuser= mysql_real_escape_string($_POST["susername"]);
$sql2 = "SELECT username FROM users WHERE username= '$user'";
$rs = mysql_query($sql2, $conn)
or die ('Problem with query' . mysql_error());
$num_rows = mysql_num_rows($rs);
if(isset($_POST['username'])) {
if($num_rows != 0){
$userErr = "Username already exists";
}
}
}
}
mysql_close($conn);
}
?>
this is what i got in the form:
<label>Chosen Username:</label> <input type="text" name="username" value="<?php
echo $userErr;?>"/><span class="error">* <?php echo $userErr;?></span><br />
<label>Password:</label> <input type="password" name="password" value="<?php
echo $passw;?>"/><span class="error">* <?php echo $passErr;?></span><br />
<br />
<label>Password confirmation:</label> <input type="password" name="passconfirm" value="<?php
echo $passw1;?>"/><span class="error">* <?php echo $pass1Err;?></span><br />

Answer 1

To acheive what your looking for, instead of using
$num_rows = mysql_num_rows($rs);
if(isset($_POST['username'])) {
if($num_rows != 0){
$userErr = "Username already exists";
}
}
replace it with
$num_rows = mysql_num_rows($rs);
if(isset($_POST['username'])) {
$userErr = "Username field cannot be empty";
}elseif($num_rows > 0){
$userErr = "Username already exists";
}
That way if the Username field is empty, it will error out. And this will also achieve your goal of checking the database to see if the user exists or not.

Answer 2

You need to know if request with username return something so error messsage else do insert new user,
$sql2 = "SELECT * FROM users WHERE username= '$user'";
$rs = mysql_query($sql2, $conn) or die ('Problem with query' . mysql_error());
if($num_rows=mysql_fetch_array($rs)){
$userErr = "Username already exists";
// ....
} else{
// $SQL = "INSERT INTO users SET ...
}