Add constants for available string values

[kavnik]

There are no enums in PHP, but it might be useful to add constants for available string values.

For example, in sendChatAction there is a parameter action with available values:
typing, upload_photo , record_video etc.

For this values it's possible to add constants in SendChatAction class:
const TYPE_TYPING = 'typing';
const TYPE_UPLOAD_PHOTO = 'upload_photo';
...

IDE will allow you to avoid typos and will give a prompt with autocomplete.

[unreal4u]

Hi @kavnik : This is certainly possible and also a good idea 👍

You can shoot in a PR or wait until I got time to implement it (which will possibly be tonight)

[unreal4u]

Implemented in v3.6.0!