database connection question

Hi,
a function display_categories() in display.php tries to connect to a
database with the function db_connect() residing in db_connections.php.
However, this results in the error "mysql_query(): supplied argument is not
a valid MySQL-Link resource ". I still can't figure out the error in my
code. Can someone enlighten me? Thanks!

Best Regards,
Edmund Yap
***********
display.php:
************
<?
include ("db_connections.php");
function display_categories()
{
  db_connect();
  echo $mysql_link;
  $query = "select * from ProductCategory order by ProductCategoryName";  //
mysql_query(): supplied argument is not a valid MySQL-Link resource...
  $mysql_result = mysql_query($query, $mysql_link); // mysql_fetch_row():
supplied argument is not a valid MySQL result resource in...

  while($row = mysql_fetch_row($mysql_result))
  {
   //blar blar blar
   }
}
?>
********************
db_connections.php:
********************
<?
function db_connect()
{
 $mysql_link = mysql_pconnect("localhost", "username", "password");
 if (!$mysql_link)
    return false;
 if (!mysql_select_db(("products"), $mysql_link))
    return false;

 return $mysql_link;

}
?>