Need help opening up a file and replacing text between 2 delimiters

I will have a PHP file

Within that PHP file I'll have something like this:

<!-- #### Start of my code -->

<!-- #### End of my code -->

I want to write code that replaces everything between those 2 delimeters in the file

I'm just not sure where to start with the pattern matching - seems so complex to me right now

I'd like to make water tight as possible - I want to have several of these in the same file

Any code to help me start would be great

Thanks

OM