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.
But PHP wouldn't be here today if it wasn't for Meta and it's support.
Actually, FaceBook worked against WordPress and the adoption of PHP because a number of people that could have used a WP instance to blog or to market a product started using a FB page instead. Ecommerce went from self hosted (Magento, Woocommerce, Prestashop) to hosted or to Amazon and also FB.
Wordpress did nothing to help further PHP other than adoption (which is still important, but not as important)