Site was unable to contact the Apache Solr server.

For those who are having the mentioned error in Drupal and running your Apache/PHP/MySQL in Windows Server, even though you can open up http://localhost:8983/solr/admin, this error can be rectified easily. The main reason this problem occurred is due to SVN checkout command-line.

To solve this problem, you do not actually need to use the command:

svn checkout -r6 http://solr-php-client.googlecode.com/svn/trunk/ SolrPhpClient

Once you have downloaded the SolrPhpClient from http://code.google.com/p/solr-php-client/ and placed it in your apachesolr folder, all you have to do is download the files from http://solr-php-client.googlecode.com/svn/trunk/ one by one and saving them in to folders respectively. For example, if you are downloading /trunk/Apache/Solr/Document.php, make sure you save the file as x:\SolrPhpClient\Apache\Solr\Document.php, and so on.

Once you have downloaded all of the files, just copy the SolrPhpClient which you created and downloaded the files in it, and paste it in /modules/apachesolr to overwrite the files.

Restart your Apache Solr server and try to do the settings in your Drupal. If you did it correctly, Drupal should be able to contact Apache Solr.