I'm using PHP to request some stuff using an external SOAP api service that does NOT have an SSL installed. During testing on a development server which is also not secured with an SSL, everything is fine.
My question is: if I install an SSL certificate on the site I am running the php SOAP requests, would my https site accept the response back from the http web service or would it flag it as insecure?
Short answer: Yes you can.
You are creating a facade pattern where your service hide another service call under its own facade.
Is it secure? Well it depends on the information you are requesting from the other site. If your web service requested weather data then there is no problem using HTTP if it was sensitive private information then HTTPS is a must. HTTP and HTTPS are identical in nature nothing special between them except that HTTPS is normal HTTP traffic encrypted using PKI certificates.
If your code does not check for security I dont think this is going to raise an exception at all. What you are doing is not exceptional from a technology view point but it could be a different situation depending on the information exchanged.