OOP issues migrating to php5

Hi, my provider upgraded server's OS (FreeBSD from 5.4 to 6.2), mySQL (5.0.x) and PHP (from 4 to 5.2.x).
Now my application don't work anymore and I'm becoming crazy finding why.

I have a page, that loads data from the DB using classes:
<?
require_once("../settings/impostazioni.php");
require_once("../libs/db_obj_class.php");

$table_name = "banner";
$db = new db_obj($table_name);
$db->search();
$db->assign_first_record();
echo "banner1=".UPLOAD_DIR."/banner/".$db->banner1."&banner2=".UPLOAD_DIR."/banner/".$db->banner2;
?>

Impostazioni.php contains the DB informations and db_obj_class.php contains the Classes and Functions.

When I build the reference of the class
     $db = new db_obj($table_name);
it should pass the table name to the function

public function db_obj($table_name,$db_connection=NULL)
{
    $this->_exists=false;
    $this->_records=array();
    $this->_table_name=$table_name;
   
     if ($db_connection)
         $this->_db_connection=$db_connection;
     else
         $this->db_connect();
         $this->table_list_fields();
}

But that value isn't passed and when I call the Search function
     $db->search();

function search($where=null,$order=null)
{
     $query = "select * from ".$this->_table_name;
     if ($where)
         $query.=" where ".$where;
     if ($order)
         $query.=" order by ".$order;
     if ( ! $res = $this->query($query) )
         return false;

     $n_res = mysql_num_rows( $res );
     for ( $i=0; $i < $n_res; $i++ )
          {
          $this->_records[$i] = new db_obj( $this->_table_name, $this->_db_connection );
          $this->_records[$i]->_exists = true;
          $this->_records[$i]->fetch( mysql_fetch_row( $res ) );
          }
}

function query( $query )
{
     echo $query."<BR>";
     $res = mysql_query( $query, $this->_db_connection );
                  
     $this->debug_notice( $query );
     $this->debug_error( mysql_error() );
                  
     if ( mysql_num_rows($res) ) {
         return $res;
     } else {
         return false;
     }
}

I notice that the table name is empty in the $query (select * from) and it gives me the error:
Warning: mysql_query(): supplied argument is not a valid MySQL-Link resource in /usr/local/psa/home/vhosts/naba.it/httpdocs/sito_2006/libs/db_obj_class.php on line 53 (that's the row $res = mysql_query($query, $this->_db_connection); )

The error is the same even if I write the table name directly in the query ($query = "select * from banner";

I just can't understand why the table name isn't passed in the class and why the query doesn't work even if I write it directly.

Thanks in advance
Roberto