For long running scripts which may be set to a longer timeout in the PHP script, be aware that the IIS connection timeout settings do not restart when you initiate a SOAP call even though you are using a new web address than your current running script.
Because of this, we noted a SOAP call that seemed to not return headers from a Web Service. It was found to be related to IIS timing out. This was in IIS 7.5. The resolution was to increase the connection timeout for IIS at the website/domain level.
There were no faults or exceptions captured when this occurred so your only indicator is that you have no header information.