Suppose you have the following html select statement
Give name to your select.
in your PHP, you will do:
$_POST['selectedValue'];
if I were you, I would prefer a switch-case incase, there are more than 2 conditions.
Example:
switch($_POST['selectedValue']){
case 'Newest':
// do Something for Newest
break;
case 'Best Sellers':
// do Something for Best seller
break;
case 'Alphabetical':
// do Something for Alphabetical
break;
default:
// Something went wrong or form has been tampered.
}