C'omo puedo verificar si un usuario existe o no en una Base de Datos MySQl

[rashid]

$query = "Select * From Registration where username ='".$username."'";
//echo $query;
$result = mysql_query($query);
$row = mysql_fetch_array($result);
$dbpassword = $row["password"];

if ($dbpassword==NULL){
echo " <script>document.location.href='/ferre/error.php?err=1'</script>";
}else{
if ($mypassword == $dbpassword){
$query = "update Registration set password='".$npassword."' where username ='".$username."'";
$result = mysql_query($query);
if ($result){
echo " <script>document.location.href='/php/registrado.html'</script>";
}else{
echo " <script>document.location.href='/php/error.php?err=3'</script>";
}
}else{
echo " <script>document.location.href='/php/error.php?err=2'</script>";
}
}