You misinterpreted my reference to extensions (or maybe I wasn't clear enough). I was refering to file extensions (.html, .shtml, .php, etc).... not extension dll's. You need to look at the extension mapping setup on your IIS installation and make sure that the .shtml extension is recognized as a PHP enabled file. If it's not, the PHP engine will never see the included code and the code will be printed to the screen as plain text. Take a look at the Win32 Installation section of the PHP manual for more detailed information on mapping extensions to PHP in IIS.
Regarding trying to add ssinc.dll into PHP, as you said, it doesn't work. The only extensions you can load into the PHP interpreter are PHP extensions (php_*.dll). And those PHP extensions must be compiled against the same build as the interpreter. For example, you cannot load 4.0.5 extension modules into 4.0.6 and vice-versa.
HTH.
-geoff