On a quick search of PHP skills wanted on LinkedIn jobs I found the following large companies looking for it:
Apple,
Cloudflare,
Vimeo,
Box,
Slack,
Tesla,
Etsy,
Wayfair,
Shopify,
Facebook,
Etc
Clearly WordPress and Magento based businesses. Add in Facebook, Weebly/Square, and you realize that PHP probably drives more page views and powers more domain named sites than any other.
How are Vimeo and Slack Magento or Wordpress businesses? Etsy also most certainly does not primarily use those.