title: Detect file change in PHP without polling
body: <p>I'm trying to detect when a particular file is modified without polling the contents of the file looking for changes.</p>
  
  <p>The server runs Mac OS X, so <code>inotify</code> is not available. My server allows for the use of <code>exec()</code>.</p>
  
  <p>How can I achieve this?</p>