Reducing PHP Memory Use on a Shared Host

Okay, so I have a shared host with a very busy phpBB forum, which means that PHP is eating a lot of memory and running me afoul of the shared resource limits. I’ve put a lot of effort into optimising the forum and trying to make as many pages as possible cacheable via CloudFlare (for example, most guest pages are now cached for a while, since they aren’t customised in any way), as well as trying to speed up database queries etc. to reduce the amount of time from a user connecting and my PHP scripts exiting to be as low as possible.

Anyway, it’s still not enough, and I’m looking at what other options I have to try to reduce memory usage. This page mentions loading Zend Optimizer; I’m wondering if anyone thinks this is worth doing, and if so what is the exact line that would be required in phprc file?

I realise I really need to consider getting a VPS so that I have more/dedicated RAM to work with, but that means finding a way to pay for it, so while I’m sure I will switch in the long-term, I’m looking for anything that might help me in the mean-time.

My site has around 100,000 page views per-day, and even though CloudFlare is caching more than 80% of file-requests, memory is still tight, especially on the weekends.

Other things I’ve already done include using InnoDB for my database to reduce table-level blocking, I’m also using Google as my default forum search.