While I share your skepticism for PHP the language, PHP is a very mature platform with well understood deployment modes.
As an example, I run one of the applications mentioned here for several customers in chrooted read-only no-exec environments without fuss. The platform gives me good tools for instrumentation and logging in order to work proactively withever operations issues that arise.
Even if your (or mine) favourite language had a community large enough to develop great webmail tools, very few has a deployment story to match.