Voted you up because you basically just described the situation I'm in now. While successfully in production, the web servers were crashing multiple times a da

Voted you up because you basically just described the situation I'm in now. While successfully in production, the web servers were crashing multiple times a day (still Apache-modphp). So, I started to work on the PHP upgrade from 4-5 to get onto php-fpm + Nginx to stabilize the site. Lo and behold, the legacy db abstraction code (poor man's ORM) was not playing well with later PHP releases so I started down the path to patch what I could. I successfully upgraded PHP, fixed a few of their outstanding issues, but I was weeks into this project and had not completed any of the new feature sets on my plate yet. Looking into the horizon I imagined months of wrestling with legacy code and the deeper I went (there was a SOAP layer I hadn't even looked at yet, they wanted REST/JSON) the more worried I became. We evaluated a bunch of options and opted for a re-write. We're still deep into it but there is light at the end of the tunnel and I'm pretty sure it will be worth it in the end.