Scusa non ho ben capito dove devo mettere le cose

io ho scritto questo ma è sbagliato, saresti così gentile da postarmi la cosa corretta così cerco di capire bene come funziona? Grazie mille

codice:
<?php
header('Content-Type: text/xml');
?>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Post</title>
</head>
<body>
<div id="xmldata"></div>
<script type="text/javascript"> 

function readyAJAX() { 
try { 
    return new XMLHttpRequest(); 
} catch(e) { 
    try { 
        return new ActiveXObject('Msxml2.XMLHTTP'); 
    } catch(e) { 
        try { 
            return new ActiveXObject('Microsoft.XMLHTTP'); 
        } catch(e) { 
            return "Hai bisogno di un altro browser."; 
        } 
    } 
} 
} 

function mostraInfo(data)
{
var requestObj = readyAJAX(); 
var url = "mostra_utenti.php"; 
var params = "q=" + data;
requestObj.open("POST",url,true); 
requestObj.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); 
requestObj.send(params); 


var serverResponse = requestObj.responseXML;
   var header = serverResponse.getElementsByTagName("user");

   htmlString = "<table><tr><th>Nome</th><th>Cognome</th><th>Username</th><th>Password</th><th>Email</th></tr>";
		
   for (i=0; i<header.length; i++)
   {
      htmlString += "<tr>";
      if (window.ActiveXObject)
      {
         htmlString += "<td>" + header[i].childNodes[0].text + "</td>";
         htmlString += "<td>" + header[i].childNodes[1].text + "</td>";
         htmlString += "<td>" + header[i].childNodes[2].text + "</td>";
         htmlString += "<td>" + header[i].childNodes[3].text + "</td>";
         htmlString += "<td>" + header[i].childNodes[4].text + "</td>";
      }
      else
      {
         htmlString += "<td>" + header[i].childNodes[0].textContent + "</td>";
         htmlString += "<td>" + header[i].childNodes[1].textContent + "</td>";
         htmlString += "<td>" + header[i].childNodes[2].textContent + "</td>";
         htmlString += "<td>" + header[i].childNodes[3].textContent + "</td>";
         htmlString += "<td>" + header[i].childNodes[4].textContent + "</td>";
      }
      htmlString += "</tr>";
   }
   htmlString += "</table>";

   document.getElementById('info').innerHTML = htmlString;    


}
</script> 
<form>
Seleziona Dipendente:
<select name="posti" onChange="mostraInfo(this.value)">
<?php



         


$db_host = "localhost";
$db_user = "administrator";
$db_password = "administrator";
$db_database = "ticket";

//Seleziono quelli che sono i dipendenti
$connessione = mysql_connect($db_host,$db_user,$db_password);
mysql_select_db($db_database, $connessione);

$query = "SELECT * 
FROM teatro 
WHERE disp='true'";
$result = mysql_query($query);
while($riga = mysql_fetch_array($result)){
echo "<option value='$riga[id]'>fila $riga[fila] posto$riga[posto]</option>";


$xml = new DomDocument('1.0'); 
    $info = $xml->createElement('informations'); 
    $info = $xml->appendChild($info); 

    while ($row = mysql_fetch_array($result)) 
    { 
        $firstname = $xml->createElement('firstname'); 
        $firstname = $info->appendChild($firstname); 
        $value = $xml->createTextNode($row['user_firstname']); 
        $value = $firstname->appendChild($value); 
             
        $lastname = $xml->createElement('lastname'); 
        $lastname = $info->appendChild($lastname); 
        $value = $xml->createTextNode($row['user_las    tname']); 
        $value = $lastname->appendChild($value);     

        $username = $xml->createElement('username'); 
        $username = $info->appendChild($username); 
        $value = $xml->createTextNode($row['user_username']); 
        $value = $username->appendChild($value); 

        $password = $xml->createElement('password'); 
        $password = $info->appendChild($password); 
        $value = $xml->createTextNode($row['user_password']); 
        $value = $password->appendChild($value); 

        $email = $xml->createElement('email'); 
        $email = $info->appendChild($email); 
        $value = $xml->createTextNode($row['user_email']); 
        $value = $email->appendChild($value); 
    } 

$output = $xml->saveXML();
}

?>
</select>
</form>


<div id="info"></div>
</body>
</html>