For example, stdlib.h headfile defines a function called "system()" in Linux: extern int system (const char *__command) __wur;
And we can call it using FFI extension:
<?php
$ffi_obj = FFI::cdef('int system(char *command);')
$ffi_obj->system('whoami');
?>
Then execute the php script as if we were calling the real C `system()`:
$ whoami
> root
$ php demo.php
> root