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I think this depends mainly on what the product is and who it targets, IMO. If the target market is professional in nature than it is less likely to be pirated than if it were, for example, a product that many individuals would want. If this product is in high demand, though, either way it will be pirated and you can be sure of that, regardless of what it was coded in.
Since both ASP and PHP are interpreted languages the code given to the purchaser is not obfuscated in any way. There are applications out there that do obfuscate code like this, but IMO these are easily worked around and just not necessary. When comparing two programming languages such as PHP and ASP I think the differences are moot. If we were talking about a compiled language, however, then that would be a different story altogether.
And as far as legality is concerned, I don't see one would be better than the other for licensing...