I think that WordPress is still big enough to keep PHP alive. Furthermore, the sheer number of developer that started coding web apps with PHP in year 2000 plus minus 5 years is large enough to give PHP a critical mass for the next 20 years.
WordPress is keeping PHP alive now
But PHP wouldn't be here today if it wasn't for Meta and it's support.
Is Automattic contributing back to PHP? I think that WordPress benefits because PHP is available, but does not significantly contribute to PHP development.