PHP is popular because of it's simplicity, low cost (like zero) of entry and because of Wordpress.

But the pay rate for PHP developers is miniscule because every Pakistani and Indian is a PHP expert for $1/hr. Try to post a $20 professional website job on Elancer - and in 10 minutes you'll get 20 lancers eating each other for your "business".

So "thriving" is to be taken with a grain of salt.