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.

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.

this file is for inclusion only -> exit now