I'm the following is not very scientific, but just to give you an idea of where I'm coming from.
Firstly comparing just the numbers of jobs (even tho this is probably a bad time of year near the holidays):
PHP Jobs, 100k/year minimum: 3 added today, 7 the day before https://www.seek.com.au/php-jobs/in-All-Melbourne-VIC?salary...
C++ Jobs, 100k/year min: 0 today, 1 yesterday https://www.seek.com.au/c%2B%2B-jobs/in-All-Melbourne-VIC?sa...
And some of those aren't even C++ jobs, switching sorting from date to relevant gets you C++ jobs from 14 days ago that would already have moved along in their hiring process.
As a cherry picked example, dropping down to 80k/year there's this Embedded C role that requires specific experience in card payment software: https://www.seek.com.au/job/33166443?type=standout&tier=no_t...
To me those requirements seem stricter than what i see in the PHP jobs (experience with MySQL + some form of MVC framework). And on top of that it's paying less.
Another example is the finance job I applied at that was paying 70-90k. But to get in the high end of that range you would need previous stock trading software experience. They also wanted 5 hours of my time for the interview process, but that's a separate topic.