Those functions provides means to executes commands on the system itself, and means secure such commands.
No external libraries are needed to build this extension.
There is no installation needed to use these functions; they are part of the PHP core.
This extension has no configuration directives defined in php.ini.
This extension has no resource types defined.
This extension has no constants defined.
These functions are also closely related to the backtick operator. Also, while in safe mode you must consider the safe_mode_exec_dir directive.