This checks the name of the PHP file currently executing.
If it is crackme.php , then the program dies before it gets to important code,
because you didn't intend for anyone to execute this file on its own.
If it is included in some other PHP file,
then the $PHP_SELF variable will be something other than crackme.php,
and the program will continue to execute.

“include secure” makes use of the built-in PHP functions eregi( ).

01 <?php
02 
03 /* --- uncomment for deployment
04  
05 // exit script if own filename is in URL
06 if(eregi(end(explode('/',__FILE__)), $_SERVER['PHP_SELF']))
07     exit;
08  
09 --- */
10  
11 // this is for demonstration only - use above line to protect your include scripts
12 if(eregi('include-secure', $_SERVER['REQUEST_URI']))
13     echo '<p>this file is for inclusion only -> exit now</p>';
14  
15 ?>