That's assuming that most profanity is directed at the language. It might be the case, but one may also be annoyed by a codebase, by an algorithm, by a bug, by a co-worker, by the business reason for the commit, or by something totally unrelated.

There can also be humourous swearing, and swearing for emphasis (see DHH), neither of which convey annoyance at all.

Hypothesis A: PHP projects tend to be smaller and simpler, with fewer interactions with other code/requirements/people, and therefore fewer opportunities for annoyance.

Hypothesis B: PHP is actually a very simple and close fit to its use-case, of unambitious webapps, and so often the tool just does its job, and disappears. There's no space to swear at a tool that you don't notice.