Ajax - porovnání políčka formuláře se záznamem v DB

<form action=<?php echo 'Zakaznik.php?akce=prihlasit';?> method="post">
  Uživatelské jméno: <input type="text" id="jmeno" value="" name="username" onkeyup="overUsername()"><br>
  Heslo: <input type="password" name="heslo"><br>
  <input type="submit" name="odeslat" value="Přihlásit">   
</form>

<script>

function overUsername(){



var XMLHttpRequest = false;
if (window.XMLHttpRequest){ // vytvoreni objektu XMLHttpRequest v novych prohlizecich
XMLHttpRequest = new XMLHttpRequest();
} 
else if (window.ActiveXObject){     // v IE5 a IE 6 se pouziva ActiveXObject
XMLHttpRequest = new ActiveXObject("Microsoft.XMLHTTP");
}

 


var input = document.getElementById("jmeno");
var cilovyDiv = document.getElementById("cilovyDiv");

if(XMLHttpRequest) {
XMLHttpRequest.open("POST","zpracuj.php", true);
XMLHttpRequest.setRequestHeader('Content-Type',
'application/x-www-form-urlencoded');

XMLHttpRequest.onreadystatechange = function(){
if (XMLHttpRequest.readyState == 4 && XMLHttpRequest.status == 200) { 
var obsah=XMLHttpRequest.responseText;      //odpoved serveru

if(obsah.responseText == "ne"){
cilovyDiv.innerHTML = "<div>ZMENTE JMÉNO.</div>";
}
if(obsah.responseText == "ano"){
cilovyDiv.innerHTML = "<div>VOLNÉ.</div>";}
}
}
XMLHttpRequest.send("jmeno="+input.value);}
}

</script>

<?php
include "DB_spojeni.php";

if($_REQUEST["username"]){

 
$prom = $_REQUEST["username"];

 $dotaz = "SELECT zakaznik_login FROM zakaznik WHERE zakaznik_login = '" . $prom . "';";
    $vysledek = mysql_query($dotaz) 
          or die(mysql_error());

if (mysql_num_rows($vysledek) != 0) 
{echo "ne";}

else
{echo "ano";}

}
?>