The Solr extension allows you to communicate effectively with the Apache Solr Server in PHP.
The Solr extension is an extremely fast, light-weight, feature-rich library that allows PHP developers to communicate effectively with Solr server instances.
Versions 1.x of The PECL Extension supports Apache Solr Server 1.3-3.x
Versions 2.x of The PECL Extension supports Apache Solr Server 4.0+
There are built-in tools to add documents and make updates to the solr server.
It also contains tools that allows you to build advanced queries to the server when searching for documents.
For more up to date information see: http://pecl.php.net/package/solr
"It [the Apache Solr PHP extension] effectively simplifies the process of interacting with Apache Solr using PHP5 and it already comes with built-in readiness for the latest features added in Solr 3.1 ..."