According to me, what's happening is that when your page is accessed over HTTPS, the Load balancer performs SSL offloading and requests your content on the non-SSL port 80. The webserver and php don't translate that content is being accessed over https://
The easiest fix for this issue is to have your Amazon's ELB send a de-fanto X-Forwarded-Proto HTTP header, which can be later used to figure out which protocol is being used by the client.
For Apache2, you can use the following command line:
SetEnvIf X-Forwarded-Proto "^https$" HTTPS