PHP remains one of the most widely deployed languages on the web, and a large share of the world's existing sites and applications run on it — including most WordPress, Drupal and Magento installations. Firms listing PHP are often maintaining and extending systems that already exist rather than starting new ones.
That makes the relevant question about the specific codebase rather than the language. Ask which framework and which version: a modern Laravel or Symfony application and a bespoke PHP system written a decade ago are different jobs requiring different people. For legacy work, ask what the upgrade path looks like and what it costs, because an unsupported PHP version is a security problem with a deadline.