title: How can I prevent to include same file twice in an included file?
body: <p>In a main <code>index.php</code> file, <code>header.php</code> is included. Both files must insert <code>session.php,</code> <code>database.php</code> etc. to run. But when header.php is included, there are two same <strong>require/include files</strong>, so there are two same functions, queries. This cause some problems in the file.<br>
  What is the solution of this problem?</p>