I want to product ordered by popularity

[charge]

Hi:

I want to product ordered by popularity,and  put sort=count into

 /index.php?id_product=1&controller=product

 

Here my code

 

$link=mysql_connect($_DB_SERVER_,$_DB_USER_,$_DB_PASSWD_);

mysql_select_db($_DB_NAME_,$link);

 

mysql_query($str,$link);

 

$str="update ps_product set count=count+1 where id_product='$id_product'";

 

How to do？Tanks a lot！