This directory (PHP_SQL) contains three PHP files that are used by the Multifocal-plane Virtual Microscope's Viewer to access the Virtual Microscope's SQL database when the user requests a new virtual slide. The Viewer (running on the client's computer) passes the virtual slide's number (an integer) to each of the PHP files (running on the server), and the PHP files return the requested metadata about the slide (obtained from the SQL database) to the Viewer. These PHP files should reside on the server in a directory named "PHP_SQL" that is at the same level as the directory containing the HTML & javascript files.