Judging from the error, one of your php configuration files has something like:

extension=/usr/lib/php5/20121212/solr.ini

in it. This tell php to try and load that as a shared library file, but it is an ini file not a so Find the file that contains that line and remove it. What's the output of:

grep -Hrv ";" /etc/php5 | grep -i "extension="