var xmlhttp = null;

function checkUser( sUrl )
{
    var sUser = prompt( 'Please Enter Your Username', '' );

    if ( sUser )
    {
        if ( sUser.length == 0 )
        { 
            document.getElementById( "xyz" ).innerHTML = "";
            return;
        }
        if ( window.XMLHttpRequest )
        {
            // Code for IE7+, Firefox, Chrome, Opera, Safari
            xmlhttp = new XMLHttpRequest();
        }
        else if ( window.ActiveXObject )
        {
            // Code for IE6, IE5
            xmlhttp = new ActiveXObject( "Microsoft.XMLHTTP" );
        }
        else
        {
            alert( "Error: Your browser does not support XMLHTTP!" );
            return;
        }

        var url="CheckUser.php?q=" + sUser + "&u=" + sUrl;
        xmlhttp.open( "GET", url, false );
        xmlhttp.send( null );

        if ( xmlhttp.responseText == 'true'             ||
             xmlhttp.responseText == 'true\n'           ||
             xmlhttp.responseText == 'true\u000d'       ||
             xmlhttp.responseText == 'true\u000a'       ||
             xmlhttp.responseText == 'true\u000d\u000a' )
        {
            document.location = sUrl;
        }
        else
        {
            alert( "Error: Invalid username!" );
        }
    }
}