PHP 5.5 appears to be available on Amazon AMI version 2013.09 (you have 2013.03).
Apparently, all you need to do is run 'yum update' and then reboot.
http://aws.amazon.com/amazon-linux-ami/2013.09-release-notes/
Installing and upgrading help
upgrading moodle 2.62 to moodle 2.73
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