I'm not sure I'd agree. By and large employers exist in language silos and will only hire devs with experience in their specific language, this means that there are distinct markets for different languages. That effect is amplified that entire sectors focus on specific languages.

So for example Java in London tends to be pretty well paid because all the investment banks hire for it heavily, this has also pushed up Java salaries in non-banking role.

The best PHP dev with great negotiating skill probably still makes less than an average Java dev at a bank.