I've seen companies with job postings for Drupal developers that require applicants to have expertise with PHP, but no experience with the platform. Drupal is based on PHP, and any developer working with it must know that programming language. However, there's so much more to Drupal than PHP, and the platform and community have numerous standards and practices that aren't involved in knowing PHP. As a result, many companies hire competent PHP developers who, despite their best intentions, didn't use Drupal effectively from the outset.