I'm sure having some PHP code at a company like Facebook isn't deterring any serious candidates. Working for a company with almost a billion users who've created software like Cassandra, Thrift, HipHop and Haystack and working on writing loosely coupled systems which will run across thousands of machines; analyzing 10s of PB of data every day with Hadoop, there are definitely very specific and challenging tasks which would attract tons of talent. I think having a PHP codebase there isn't indicative that your new job would be sitting and writing crappy PHP code templates all day and wanting to kill yourself because it is so boring. Just my 2c.