PHP include before or after Apache SSI?

In which order do the two execute? I'm trying to work out the best way to integrate PHP backend code with an HTML interface in the cleanest way possible - I don't really want to copy & paste the PHP into the HTML because it will make maintaining both the PHP & the HTML more complicated. What I'd really like is to save the PHP backend separatly and link them into the HTML at the appropriate place using SSI. Can someone offer some guidance on the best way to do this, I'm getting confused as to how's best to do it. There are lots of sites out there that have lush UI's and fairly complex PHP yet they must somehow manage the two separately - after all they're usually done by different teams so how do they do it? Thanks