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.

download options for file handling / include secure

you can download the source as PHP file,

open the file as plain text in a new window or

get the code in a textarea to copy it.