hmm, a different way to what I normally see on servers, usually,
AddHandler application/x-httpd-php5 .php
I guess 1and1 do it differently - they seem to make a habit of aberrations, with consequent issues for their users.
In other words, BizStore works just fine on PHP5, always has - I have tested BizStore where PHP5 is standard on the server, both hosted and local, and also where it is an option via htaccess, as above.
Just not on PHP5 the way that 1and1 chooses to implement the option - since PHP4 is no longer supported, they really should be using PHP5 on their servers, as first choice, not some ad hoc htaccess implementation.
Perhaps their htaccess implementation should be,
AddHandler x-mapp-php5 .php
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):