Are you using some sort of version control software for your php/css/html?
This sort of situation is one of the more compelling examples of why that sort of thing is needed IMO. Put your entire site/sites into source control, check it out at your new server, get a recent copy of the mysql database up and going, and then test it before the changeover.
In the end, the final transfer can be what everyone else has already said, but to test it out in advance removes quite a bit of doubt and lowers your blood pressure to boot.
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