tl;dr:

I've found myself hosting a bunch of sites running on the same web framework (symfony 1.4). Would it be helpful if I moved all of the shared library code into the same directory and shared it across the sites?

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I see some advantages to this:

• Each site takes up less disk space
• Library updates (an unlikely scenario) can take place across all sites

I also see some disadvantages, mostly in terms of a single point of failure and the inability to have sites using different versions of the framework.

My real concern, though, is performance. I hypothesize that I will see a performance increase, since the PHP code will already be cached for all sites when they call the framework. Is this a correct hypothesis?