Alas, I don't know anything about PHP/web interfaces, but we run reports through our app and of course the theory is the same. You can create a report and have the user directly enter the parameters into the report, then the report can fetch the data and display, or you can have a form that lists possible values for the parameters, and update the parameters in the report and the report fetch the data and display, or you can have the users select/enter the values of the parameters fetch it through the app and pass it to the report. There aren't very many dlls that you need to include in your project, and while I don't know what they off-hand, Business Objects should be able to help with what is needed for web developing.
I know that my answer is vague, but I am pretty sure that if you own a copy of CR, that you have everything that you need to create/post/run reports on a web server and to access it programatically. I would start at SAP/Business Objects to see what they have to say...also .Net forums here might be of help.
HTH
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