Thanks to Lester, finding this discussion , all the words spent about
using FBCLIENT.DLL compiled in VC6/VC9 for PHP 5.3 (and any other
similar question) seems to belong to the past (actually Thursday, March
27, 2008 8:08 AM )
More than a year ago.

http://social.msdn.microsoft.com/forums/en-US/vcgeneral/thread/8042a534-aa8b-4f99-81ee-e5ff39ae6e69/

Ciao.
Mimmo.