Как извлечь имена пользователей из базы

<?php
//  Developed by Roshan Bhattarai 
//  Visit http://roshanbh.com.np for this script and more.
//  This notice MUST stay intact for legal use

//this varible contains the array of existing users
$existing_users=array(''); 
//value got from the get metho
$user_name=$_POST['user_name'];
//checking weather user exists or not in $existing_users array
if (in_array($user_name, $existing_users))
{
	//user name is not availble
	echo "no";
} 
else
{
	//user name is available
	echo "yes";
}
?>

<?php
$sql="SELECT username FROM modx_web_users";
$result=mysql_query($sql);
while($row=mysql_fetch_array($result)) {
       $return[] = $row;
   }

$existing_users=$return[]; 
//value got from the get metho
$user_name=$_POST['user_name'];
//checking weather user exists or not in $existing_users array
if (in_array($user_name, $existing_users))
{
	//user name is not availble
	echo "no";
} 
else
{
	//user name is available
	echo "yes";
}
?>

<?php
$sql="SELECT username FROM modx_web_users";
$result=mysql_query($sql);
while($row=mysql_fetch_array($result)) {
       $return[] = $row;
   }
$existing_users=$return; 
//value got from the get metho
$user_name=$_POST['user_name'];
//checking weather user exists or not in $existing_users array
if (in_array($user_name, $existing_users))
{
	//user name is not availble
	echo "no";
} 
else
{
	//user name is available
	echo "yes";
}

# add the output 
echo var_dump($return);
echo "<br/>".$_POST['user_name'];
?>

<script language="javascript">
$(document).ready(function()
{
	$("#username").blur(function()
	{
		$("#msgbox").removeClass().addClass('messagebox').text('Проверка...').fadeIn("slow");
		$.post("scripts/user_availability.php",{ user_name:$(this).val() } ,function(data)
        {
		  if(data=='no') 
		  {
		  	$("#msgbox").fadeTo(200,0.1,function() 
			{ 
			  $(this).html('Этот логин занят').addClass('messageboxerror').fadeTo(900,1);
			});		
          }
		  else
		  {
		  	$("#msgbox").fadeTo(200,0.1,function()  
			{ 
			  $(this).html('Логин доступен для регистрации').addClass('messageboxok').fadeTo(900,1);	
			});
		  }
				
        });
 
	});
});
</script>