sudo su

Install dependencies:

apt-get install libxml2-dev libcurl4-openssl-dev

Install by pear

pear install pecl/solr

Or compile from source

cd /opt
wget http://pecl.php.net/get/solr-1.0.2.tgz
tar -xvf solr-1.0.2.tgz && cd solr-1.0.2
./configure
make
make install