I use Solr professionally to accomplish different full text search and analysis abilities that I can't accomplish with MySQL alone. To interact with Solr you make HTTP requests to specific configured URL endpoints with either GET query parameters or with an XML POST body. The Solr project had example PHP code to make different requests, but nothing that really seemed to be appropriate to drop in and use in my project. So, I wrapped them up into a set of PHP classes with specific methods for each of the common actions (URL endpoints) to make usage simpler. With the blessing of my employer, I released this as open source on Google code:
See the WIKI for frequently asked questions and some example usage. There is also a google group for user discussion - I'm willing to answer direct email related to usage, but the community may be able to help you faster or give you better answers. Please direct questions only about PHP specific usage, for more general usage you're better off hitting the Solr / Lucene user mailing lists.
There are search plugins that use the PHP client code for Drupal, Symphony, Solar, OSCommerce, and more - written by members / companies in the respective communities. Individual downloads from the google code site have gone to over 30K (as of this writing).