fix function is already exist

[jimmydupre]

added function_exists on multiple files because if twig is stocked in include_path on php config and another place. We risk a conflict. Thank you

[fabpot]

If these functions already exist, I suppose that this should be fixed instead of hiding the problem by checking for existence (which would not guarantee that the implementation is the same).

[jimmydupre]

In my case, twig is incorporated into the include_path for general use. This is an unusual usage. However, in the virtualhost of the main domain there is a subproject (CMS) that uses twig. This causes a conflict because the functions are already declared.

[fabpot]

I understand your (unusual) setup. But this is not something that we will support for the reason explained above.