Adding these code in your php.ini file will protect your server from execution of Shells like c99,r57 or b374k.
disable_functions = php_uname, getmyuid, getmypid, passthru, leak, listen, diskfreespace, tmpfile, link, ignore_user_abord, shell_exec, dl, set_time_limit, exec, system, highlight_file, source, show_source, fpaththru, virtual, posix_ctermid, posix_getcwd, posix_getegid, posix_geteuid, posix_getgid, posix_getgrgid, posix_getgrnam, posix_getgroups, posix_getlogin, posix_getpgid, posix_getpgrp, posix_getpid, posix, _getppid, posix_getpwnam, posix_getpwuid, posix_getrlimit, posix_getsid, posix_getuid, posix_isatty, posix_kill, posix_mkfifo, posix_setegid, posix_seteuid, posix_setgid, posix_setpgid, posix_setsid, posix_setuid, posix_times, posix_ttyname, posix_uname, proc_open, proc_close, proc_get_status, proc_nice, proc_terminate, phpinfo,system,passthru,shell_exec,escapeshellarg,escapeshellcmd,proc_close,proc_open,ini_alter,dl,popen,popen,pcntl_exec,socket_accept,socket_bind,socket_clear_error,socket_close,socket_connect
safe_mode = On
register_globals = Off
display_errors = Off
allow_url_fopen = Off
allow_url_include = Off
enable open_basedir(set it to webroot path)
