Here's a proper way of installing PECL runkit on centos.
cd /usr/local/src/ git clone https://github.com/zenovich/runkit.git cd runkit phpize ./configure make make install
Now edit your php.ini
vi /usr/local/lib/php.ini
Add the following under your extension= listings.
; PECL runkit extension=sorunkit.so
Now restart Apache.
service httpd restart
That's it.