So I'm confused here.
You don't know how to actually program PHP, yet you are applying to become a PHP developer? I would think that if you are given the position they'd learn pretty quickly that you don't actually know PHP.
On the other hand if you do know how to program PHP, than write whatever and put it online. There's great value in putting it online even if its not real code such as a web store. Showing you can do it is sufficient even if you don't implement the real functionality required to operate.
As of PHP 5.5, the MySQL library has been officially deprecated. It is recommended to move to either MySQLi or PDO libraries for your mysql connectivity. See here for help choosing which interface you prefer: http://php.net/manual/en/mysqlinfo.api.choosing.php