Add the LoadModule command to your httpd.conf and point it to the php56 module from RHSCL:
LoadModule php5_module /opt/rh/httpd24/root/usr/lib64/httpd/modules/librh-php56-php5.so
This generally seems to work for me, but I have only tested basic page rendering. I wouldn't be surprised if there are some bugs related to loading a Special Collections module in the regular Apache config. After all, RH Special Collections packages intentionally install to a completely different part of the filesystem to avoid conflicts. You may want to consider using HTTPD24 from the RHSCL along with PHP56 to avoid configuration issues.