check if username exists in real-time through AJAX in php with mysql

Send your data in javascript using jQuery like this:

$.post( "check.php", { user: $("#username").val() }, function (data){
  if(data=='1'){
   //do 1
  }
  elseif(data=='0'){
    //do 0
  }
});

In your check.php get username like this

//some basic validation
if(!isset($_POST['user'] || empty($_POST['user']))
{
   echo '0'; exit();
}

$username = trim($_POST['user']);

For this to work properly you check.php must return either 1 or 0, do not echo mysql errors there or whatsoever. if any error occur, just echo 0.

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