How do you handle MySql database error messages using PHP / dealing with "unique" contraint?

<?php
 
// CODE FOR THE MAIN MODULE:
 
declareRequest('idx','first_name','last_name','username','o','f','action','password','access_careers','access_newsevents','access_full','access_seo','ID','email');
	loadClasses('BackendUser');
	global $BackendUser;
 
  if( !$BackendUser->isFullAdmin($BackendUser->getUserId()) )
  {
  	print " Access Denied ";
  }
	$strError  = '';
	$strUpdate = '';
	$tab       = 'info';
 
	$ID = ($_REQUEST['ID']) ? $_REQUEST['ID'] : 0;
 
 	switch($_REQUEST['action'])
	{
		case 'save':
			if(!$strError)
			{
				if($ID)
				{
					$BackendUser->Edit($ID,$_REQUEST['first_name'], $_REQUEST['last_name'],$_REQUEST['email'],$_REQUEST['username'],$_REQUEST['password'],$_REQUEST['access_careers'],$_REQUEST['access_newsevents'], $_REQUEST['access_full'], $_REQUEST['access_seo']);
				  forward("admin_admins_added","ID=".$ID);
				}
				else
				{
					$ID = $BackendUser->Add($_REQUEST['first_name'], $_REQUEST['last_name'], $_REQUEST['email'], $_REQUEST['username'],$_REQUEST['password'],$_REQUEST['access_careers'],$_REQUEST['access_newsevents'], $_REQUEST['access_full'], $_REQUEST['access_seo']);
 
          if($ID)
          {
          forward("admin_admins_added","ID=".$ID);
          }
        }
 			}
			break;
	}
 
  if($ID)
  {
  $arr = $BackendUser->GetUser($ID);
  }
  else
  {
  $arr['first_name']		= $_REQUEST['first_name'];
  $arr['last_name']  	  = $_REQUEST['last_name'];
  $arr['username'] 			= $_REQUEST['username'];
  $arr['password'] 			= $_REQUEST['password'];
  $arr['access_careers'] 		= $_REQUEST['access_careers'];
  $arr['access_newsevents'] = $_REQUEST['access_newsevents'];
  $arr['access_full']  = $_REQUEST['access_full'];
  $arr['access_seo']   = $_REQUEST['access_seo'];
  $arr['email']   		 = $_REQUEST['email'];
  }
?>
 
--------------------------------------------
 
<?php 
 
// CLASS FILE CONTAINING THE "ADD USER" FUNCTION
 
Function Add($first_name, $last_name, $email, $username,$password,$access_careers,$access_newsevents, $access_full, $access_seo)
{
global $link;
 
$access_careers		 = ($access_careers) ? $access_careers : 0;
$access_newsevents = ($access_newsevents) ? $access_newsevents : 0;
$access_full 			 = ($access_full) ? $access_full : 0;
$access_seo 			 = ($access_seo) ? $access_seo : 0;
 
$Query = "INSERT INTO users (first_name, last_name, email, username, password, access_careers, access_newsevents, access_full, access_seo) VALUES ('".$first_name."', '".$last_name."', '".$email."', '".$username."','".$password."',".$access_careers.",".$access_newsevents.", ".$access_full.", ".$access_seo.") ";
 
$Result = @mysql_query($Query,$link);
 
return @mysql_insert_id();
}

                                              

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