SSI.php on other websites, Not working

I've tried embedding SSI.php functions on other websites, but it doesn't work, all that shows up is a blank white screen. Using my test-forum's SSI.php, however, I am able to embed any SSI.php function, so it's not a matter of knowing how to code it, it's a matter of my SSI.php is not working properly.

I tried using a new SSI.php function by uploading it to my server, but it didn't work.

What could the problem be? My forum is heavily modded, so what other files could effect the SSI.php?