As of writing, the current pecl runkit-0.9 package does not compile with PHP 5.2+ . Below is how I got the latest installed from svn trunk on CentOS-5.4 with php-5.2.9.
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Install runkit:
svn co http://svn.php.net/repository/pecl/runkit/trunk/
cd trunk
phpize
./configure
make
make install
extension=runkit.so
Note: you would need php-pear and php-devel installed already.