Description:
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Feature request.

It is possible to disable PHP functions via the `disable_functions` directive in php.ini. However, there is no method of testing, at runtime, whether a given function has been disabled by this means,

The common way to detect whether a function has been disabled is to inspect the return value of `ini_get('disable_functions')`. This is far from optimal, especially if `ini_get()` itself has been disabled.

A function should be introduced which accepts a function name and returns a boolean value stating whether the function has been disabled. Example:

`if ( ! function_disabled('exec') ) { exec( $command ) }`

Alternatively, to avoid the double negative, it could be named something like `function_enabled()` or `function_available()`.