Memcached vs Amazon Elasticache with PHP on CentOS 7

I setup a new CentOS 7 server in Vagrant to run PHP, nginx, MySQL (MariaDB) and memcached (note the d on the end). I can get memcached to run fine, but I can't get the Amazon Elasticache .so file to be loaded by PHP. I know the file exists. How do I get PHP to load this extension, or troubleshoot why it won't?

I have followed the instructions for installing this extension here: http://ift.tt/1Co0PXn

How do I troubleshoot PHP and why it may not load an extension?