php - Using the jQuery validate plugin with the "remote" option -

i using jquery validate plugin , remote option checking existence of username input field. use code:

 remote : {       url :'ajax_php/admin/checking.php',       type   :'post',       data   : {       type:'username'    }} 

i've noticed request url has callback parameter appended, though set type request post:

http://localhost/lopoli2/ajax_php/admin/checking.php?callback=jquery15104128487491980195_1311232389069 

my php script works fine , returns true valid username , string invalid usernames. no error message appears! php file in short looks this:

$check = mysql_query("select `username` `user_tbl` `username`='".$_post['username']."' ",$conn)  or die('error in db !');  if (mysql_num_rows($check)>0){     echo("username exists"); }else{     echo("true"); } 

here's want know:

• what callback parameter for?
• how solve "display error message" problem?

try changing:

echo("username exists"); 

to

echo("false");