rrdtool-php in CentOS/RHEL 6 is built against the php 5.3 extension api, and so can't be used in later releases of PHP (neither 5.4 or 5.5).
A package could be created that supports this php extension in PHP 5.5, but it's not something I plan to do for Webtatic, as not a common request.
If you find it suitable for yourself, you can instead use the pecl installer for the php extension:
yum install rrdtool-devel php55w-pear php55w-devel
pecl install rrd
echo "extension=rrd.so" > /etc/php.d/rrd.ini