Ajit there are some articles on this very site that describe how you can implement credit-card authorization/settlement and other commerce activities.
Mail functionality is built-in... see the mail() function.
As far as larger components... what I have seen done is they are implemented in java servlets, cgi's or isapi's (IIS)... and then php can just fopen(url) to marshal their services. Very easy... platform independent and quite scalable.
Dave
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