If you want to add the Solr extension to your exisiting PHP install, you need to make sure you use the same API to build the extension as was used to build PHP or you'll get errors like:
PHP Warning: PHP Startup: solr: Unable to initialize module\nModule compiled with module API=20060613\nPHP compiled with module API=20090626\nThese options need to match\n in Unknown on line 0
To avoid this, use your existing phpize and php-config when compiling the extension (change the '/usr/local/php5/' below to whatever your existing path is.)
/usr/local/php5/bin/phpize
./configure --with-php-config=/usr/local/php5/bin/php-config
make
make test
make install