Those functions provides means to executes commands on the system itself, and means secure such commands.
These functions are available as part of the standard module, which is always available.
There is no installation needed to use these functions, they are part of the PHP core.
This extension does not define any configuration directives.
This extension does not define any resource types.
This extension does not define any constants.
Those functions are also closely related to the backtick operator.