payflowpro problem

Does anyone know if there are particular settings in php 4.3.4 that prevent payflowpro from responding properly? I'm getting 'Amount mismatch' when I run my function - which runs on other sites and ips just fine (i.e. I can move the exact code over there and it works fine) - but it doesn't seem to be even getting to verisign to begin with. All of the proper ports seem to be open as well. Since verisign uses port 443 for this, could it be an ssl or other issue? Thanks, -Mike