PHP displaying error for already used Username and empty field

Question

I want PHP to make sure the username is not already used and also check to see if the field is empty. Sorry I am a huge noob when it comes to php. Here is my code:

// Check for an Username:
    $dup = mysql_query("SELECT user_username FROM users WHERE user_username='".$_POST['user_username']."'");
        if(mysql_num_rows($dup) >0){
            $errors[] = 'Username already used.';
        }
        else{
            $un = mysqli_real_escape_string($dbc, trim($_POST['user_username']));
            echo '<b>Congrats, You are now Registered.</b>';
        }
        else {
            $errors[] = 'You forgot to enter your Username.';
            }

Answer 1

You have two else statements, which cannot be used. Also, you need to escape your $_POST data. Lastly, you should move away from mysql_* functions as that time has passed. Use either mysqli or PDO.

<?php
$sql = sprintf("SELECT user_username FROM users WHERE user_username='%s'",
        mysql_real_escape_string($_POST['user_username']));
$dup = mysql_query($sql);
if(empty($_POST['user_username'])){
    $errors[] = 'You forgot to enter your Username.';
}elseif(mysql_num_rows($dup) >0){
    $errors[] = 'Username already used.';
}else{
    $un = mysqli_real_escape_string($dbc, trim($_POST['user_username']));
    echo '<b>Congrats, You are now Registered.</b>';
}?>

An alternative, and easier flow in my opinion, would be a try/catch statement. This way you don't have to make an unnecessary database call if the username is empty:

<?php
try{
    if(empty($_POST['user_username'])){
        throw new Exception('You forgot to enter your Username.');
    }

    $sql = sprintf("SELECT user_username FROM users WHERE user_username='%s'",
        mysql_real_escape_string($_POST['user_username']));
    $dup = mysql_query($sql);

    if(mysql_num_rows($dup) >0){
        throw new Exception('Username already used.');
    }

    echo '<b>Congrats, You are now Registered.</b>';

}catch(Exception $e){
    echo $e->getMessage();
}?>