I think it might be php 5+, so I'm not sure what you would do for php 4, which is probably what you are using if you get a function undefined error. I'd update to php 5 if you can... otherwise you can try defining it:
<?
if (!function_exists('mysql_set_charset')) {
function mysql_set_charset($charset,$dbh)
{
return mysql_query("set names $charset",$dbh);
}
}
?>