mysql - Multiple database and transactions

Question

If i use two database, i have to start to transaction? Is this correct, or this code is wrong? If i make a mistake in the second query, then call rollback(), but unfortunately won't roll back the first query...

 $conn_site=mysql_connect("localhost", "us", "ps");
 mysql_select_db("site",$conn_site); 
 $conn_forum=mysql_connect("localhost", "us", "ps");
 mysql_select_db("forum",$conn_forum); 

     function begin() {

         @mysql_query("BEGIN",$conn_site);
         @mysql_query("BEGIN",$conn_forum);
     }
    function commit_reg() {
        @mysql_query("COMMIT",$conn_site);
        @mysql_query("COMMIT",$conn_forum);
    }
    function rollback(){
        @mysql_query("ROLLBACK",$conn_site);
        @mysql_query("ROLLBACK",$conn_forum);
    }
   begin();
    mysql_query("insert into users (....) or rollback();
       mysql_query("insert into forumusers (....) or rollback();
    commit();

Answer 1

mysql supports XA transactions, which allow for the two-step commit like robert suggests, but in a more formal manner. the PHP interfaces don't directly support XA transactions, so you'll have to send the transaction commands yourself as statements.