Elastic Beanstalk does not, by default, install the ElastiCache Cluster Client PHP module. This is needed to connect to an ElastiCache node cluster. Reading around, most of the instructions relate to creating an ElastiCache resource (which I assume will also install the PHP module on the Elastic Beanstalk). I want to install the PHP module without creating the resource as I want to use an existing cluster. (64bit Linux PHP5.5)
You will need to download the AWS ElastiCache client for PHP which is available from the AWS ElastiCache Console. The client also supports Auto-Discovery of the ElastiCache nodes you will just need to configure the client with the proper endpoint for your ElastiCache cluster.