Validate email with database

I have created a form and have validated everything using PHP, but can't figure out how to validate email from database. If I have the entered username in the database, I want it to display an error. I have connect.php and

just for an example -

here's how i validate password -

if(开发者_JS百科!empty($_POST['password']))
{
 if($_POST['password'] != $_POST['cpass']) 
 {
  $errors[] = 'The password and confirm password do not match.';
 }
 else
 {
  $p=trim($_POST['password']);
 }
}

here is what i'm trying to do -

$getusername = "SELECT username FROM users WHERE ($u,$username)";
if($getusername)
{
    echo 'Username is already in use.';
}
else
{
    $g=trim($_POST['username']);

}

THIS RESULTS IN A PARSE ERROR.

// first define the username from the $_POST variable
// make sure to escape the value to prevent SQL injection
$username = mysql_real_escape_string(trim($_POST['username']));

// select a user with the posted username
$sql = "SELECT username FROM users WHERE username = '$username' LIMIT 1";

// run the query
$res = mysql_query($sql) or die(mysql_error());

// see if there's a result
if (mysql_num_rows($res) > 0) {
  echo 'This username is already taken';
} else {
  // .. do stuff
}