title: what is the functionality of `/* $Id$ */` in php-src files
body: <p>It's included in many files.</p>
  
  <p>For example:</p>
  
  <p><code>
  /* $Id: php_cli.c 306938 2011-01-01 02:17:06Z felipe $ */
  </code></p>
  
  <p>Is this helping debug process or something else? Any tool related?</p>