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