May related to:
1.
This line in my web.config file was causing the issue:
<trace enabled="true" localOnly="false" pageOutput="false" requestLimit="40" />

2.
I had this same problem. Any PHP POST to another PHP page was hanging. In the process of rebuilding the web.config file from scratch, I found an error message that suggested running the AppPool Managed pipeline mode in Classic mode.
After making the above change, my PHP code is working as expected.

Reference:
http://stackoverflow.com/questions/4357636/iis7-php-http-post-hang