Auto redirect to 'index' file destroys HTTP_HOST

[MrCat]

I have multiple domain names pointing to the same site.

 

This works fine in the root folder: mysite.com and myothersite.com begin at mysite.com/index.php and I can change display settings depending on HTTP_HOST (which is either set to mysite.com or myothersite.com).

 

It doesn't work when I go one folder deeper: mysite.com/start and myothersite.com/start are always seen as mysite.com/start/index.php - destroying the HTTP_HOST variable.

 

What can I do? I'm on a shared host so can't do much in the way of config.

Any help appreciated