The @php directive declares what follows is PHP code to be embedded as is into the generated page. It also causes the generated page to have a PHP extension.

So for example Practice has:

  @PHP
  <?php
  $keys = array("A flat",  "A", "B flat", "B", "C",
  "C sharp", "D", "E flat", "E", "F",
  "F sharp", "G");