Note that it is the .conf file that controls which version of php is used. For some reason this does not seem to be available for php55. It is possible that anther package needs to be installed, containing libphp5.so.
yum install -y centos-release-SCL scl-utils && yum clean all && yum makecache
rpm -qa | grep php P=php54-php yum install -y \ php54 \ $P \ $P-devel \ $P-gd \ $P-imap \ $P-ldap \ $P-mbstring \ $P-mysqlnd \ $P-odbc \ $P-xmlrpc # and $P-opcache for php 5.5
source /opt/rh/php54/enable
mv /etc/httpd/conf.d/php.conf /etc/httpd/conf.d/php.conf-5.3 service httpd restart