You attribute high availability to the infrastructure entirely, not just a front facing programming language. Facebook has a massive architecture and only it's

You attribute high availability to the infrastructure entirely, not just a front facing programming language. Facebook has a massive architecture and only it's front-end stuff (views, data retrieval) are done in PHP. Everything else are separate services written in Java (or some other JVM language) and communicate through Thrift. PHP isn't accredited with Facebook's high availability and Facebook doesn't even serve PHP, it serves compile C++ code that was compiled from PHP.