I doubt there is anything in PHP that can do this directly. Digital signatures are only supported by Adobe Acrobat as far as I know. Since the PDF extension in PHP is built by a third party, I doubt it will recognize it. The platform is likely to be platform dependant.
There is one option you can look at. PDF documents are text based. Not really clear text, but you should be able to open them in a text editor and see the differences. Try comparing the original copy with a signed one to see what is changed in the file. Some tools can help you do this. My text editor (vim) allows me to compare two files and highlight the differences. Once you find the pattern of the signature, you can write a script to check if it’s present in the file you received.
Since digital signatures are based on a key, you won’t be able to know if it’s the right signature. You will probably only be able to know if there is a signature. It depends on what kind of security you need and who your users are.
Otherwise, there might be a call you can make to adobe acrobat to find that out, but I can’t help you in that direction.
The other solution is the most simple one: manual check. If you don’t get too much of these coming in, placing them in a cue and assign someone to verifying those signatures might just be a cheaper solution than trying to automate the process.