PrefixAllGlobals: prevent false negatives for autoloaded user-defined global functions

[jrfnl]

The PrefixAllGlobals sniff verifies that functions declared in the global namespace are prefixed with one of the whitelisted prefixes as passed to the sniff in the prefixes property in a custom ruleset.

The sniff prevents false positives for polyfills for PHP native functions - which should be named exactly as named in PHP without prefix for them to be usable as a polyfill - by checking that the function doesn't exist.

Generally speaking this works fine in 98% of all cases as PHPCS is normally run stand-alone and doesn't contain any functions defined in the global namespace.

Similarly, when PHPCS is installed via Composer, this would normally work fine as the commonly used autoload options are PSR-0, PSR-4 and classmap which are all based on code being in classes.

However, if the Composer autoload files option is used to load, for instance, a functions file declaring functions in the global namespace, this "polyfill false positive prevention" would incorrectly cause errors not to be thrown for functions declared in the global namespace which were now autoloaded via Composer.

I have now fixed this by, instead of using function_exists(), using a check against a functions list retrieved via get_defined_functions().

This change does not have unit tests as:

• Preventing false positives for polyfills is already unit tested.
• Checking that autoloaded user defined functions are not recognized as PHP native functions is not something which can be unit tested as such. This would need an integration test including a composer setup to be tested.

Based on the test case provided in the issue which originally reported this, I have confirmed that this PR fixes the issue though.

Fixes #1632

[GaryJones]

Minor docs tweak. Code otherwise good to go.

Fixed