Can I decline a PhD program I have already accepted? Can I accept a PhD offer and later withdraw from the program before it even starts? Is there an etiquette for that?

I really want to do a PhD but they have put a deadline in accepting and I am currently interviewing with companies. Please advise.
Can I accept a PhD offer and later withdraw from the program before it even starts? 


Yes, of course you can. They won't force you to enroll and they won't come after you legally. It's certainly not worth anyone's time. However, it does harm the department's planning because they might not be able to fill your spot. For more discussion, see the comments on this answer.


  Is there an etiquette for that?


Yes, the etiquette is to not do that. It's rude and you will burn bridges. A spot that could have been offered to another student may be lost. But if you're quitting to move to industry, you probably don't care anyway.