If PHP is putting you off considering working at Facebook, then it may be useful to know that there are many developers who hardly ever touch PHP at Facebook.
Most back-end services (ie, the systems that build your feed, or detect spam) are in C++. Java is used in some back-end services (including, for example, in Hadoop, Hive, HBase, and the messaging product) and for the Android app. Objective C is used for the Facebook app, Messenger, Camera, and so forth on iOS. Python is used primarily in the automation and operations space. You will also find a bunch of Ruby, Haskell, Scala, and so forth around.
I am not generally a fan of PHP and much of the ecosystem around it, but using it at Facebook is very different. I also would not say that time spent learning and using PHP is time wasted - the important things to learn typically transcend language, and learning anti-patterns / deficiencies is useful too. That is also why Facebook does not expect any particular language knowledge before you start/when you interview.
Most back-end services (ie, the systems that build your feed, or detect spam) are in C++. Java is used in some back-end services (including, for example, in Hadoop, Hive, HBase, and the messaging product) and for the Android app. Objective C is used for the Facebook app, Messenger, Camera, and so forth on iOS. Python is used primarily in the automation and operations space. You will also find a bunch of Ruby, Haskell, Scala, and so forth around.
I am not generally a fan of PHP and much of the ecosystem around it, but using it at Facebook is very different. I also would not say that time spent learning and using PHP is time wasted - the important things to learn typically transcend language, and learning anti-patterns / deficiencies is useful too. That is also why Facebook does not expect any particular language knowledge before you start/when you interview.