Integrate a webservice to an existing PHP site

Need to integrate a web service to an already existing php site. the site is used to register for personal vehicles and print receipts. Normally a vehicle owner goes to the site users to register their vehicle. the site user collects money from them for the product they want to register, registers them and print out receipts for them. now, we want all vehicle owners that wants to register with us to go the bank and pay then go to the site users who verifies their payment with the bank using the web service then print out the receipts for them.