PHP is definitely the way to go. Javascript is flawed because you need a browser to have it enabled in order for it to work. This is why I think it is stupid when people do their form validation using JavaScript.
As for writing the script, you would basically store all of your products (descriptions, quantities avail., cost, etc) in the mysql database and use HTML forms to display and update the values.
You really need to look at
http://www.sqlcourse.com for SQL query information, and
phpcomplete.com has good PHP information.