Earlier quoted context omitted.
It is a myth. High load has nothing to do with it. However if you configure apache wrongly, it will serve .php files as text. Only connection to load is if you have one broken server among N proper ones, a number of times that the broken one is hit depends on load - I.e. on low load, it may be configured so that it is never hit at all. I guess the story about apache serving php source under high load came from the id…
this was a real issue, here is the bug: https://bugs.php.net/bug.php?id=26810 there were many related bugs as well. it didn't get fixed for a while, and when it was fixed it was over a number of revisions and wasn't tagged.
That issue was fixed 3 years before (Jan 2004) and from what I can see only happened in configs which use +x bit or config values to turn php engine on and off, and didn't really depend on load on these too much either - it's just load made it more probable you will hit the right sequence in the same process randomly, but if you know what to hit, it could be done on any load. So it would be correct to say "there was a bug 3 years ago which in certain configurations made mod_php serve source after certain sequence of requests". It was never happening just "due to high load", as far as I can see. And as far as I know (and nobody so far presented any evidence to the contrary, AFAIK) Facebook case had nothing to do with it anyway. It would be really weird for Facebook to run random 3-years-old PHP in production too.