Do not allow mixture of forward and backslashes in install path

If this idea is correct. Just an idea review, tiny improvement at most.

Also, I was not sure about the "\\\\" pattern part, in which environments does it match, and if its ok also like \\install\\index.php . If not, a "\\{1,2}" pattern would be fine I think. Well, initially I was thinking about to use only a "\\", but this syntax was throwing errors in my local tests (I don't understand why), while on online testers it was fine.