Implement exists() construct

[flaupretre]

This works targets PHP 7.1. RFC is under way.

[c9s]

Probably rename exists to defined since it might be confusing with array_keys_exists?

[nikic]

@c9s There is already a defined function. http://php.net/defined

[carusogabriel]

Is there a RFC for this? I couldn’t found it 😞

[cmb69]

This seems to have some overlap with PR #3360.

[Majkl578]

At this moment I think this doesn't need a specialized language construct, that's also why I implemented it as a function optimization in #3360. Even justifying array_key_exists() is 50-50 imho, judging from Dmitry's comment in the other PR (which I can agree with, but mostly because people avoid that function now due to its performance penalty).

[php-pulls]

Comment on behalf of kalle at php.net:

Closing due to inactivity