Updated path to SSL certificate in PHP API

The PHP API wrapper was updated so that the path to the SSL cert is relative to the leftronic.php's path instead of relative to the current working directory.

Prior to the change, `getcwd()` was being used to determine the path to the SSL certificate. Using `getcwd()` requires that the CAcerts directory be in the same directory that the script is being called from.

By using `dirname(__FILE__)`, the path becomes relative to the leftronic.php file instead. The script can then be called from any directory—or in my case the script that does an `include 'path/to/leftronic.php';` can be called from any directory—and the certificate can be successfully found without any need to modify leftronic.php or changing the current working directory.

With the change, it becomes possible to use the PHP API when including the leftronic git repo into another git repo as a git submodule.