1. Monitoring and administration tools for MySQL are (better?) more polished.
2. WAY easier to find MySQL DBA's vs. PG DBA's
3. More resources in general around MySQL. Whatever problem/issue you have it's out there already.
Suggesting a code rewrite w/o any idea of what the code looks like or what the issues are is pretty awful.
PHP is a good solution for plenty of projects out there. There are plenty of high revenue operations that run on PHP. Like any other language it just needs to be well coded and maintained.
What's needed here is looking at the current state of the system, detecting obvious issues and determining the course of action based on that + expected growth.