There are two different issues here. One is what does the GPL say, and the other is what does MySQL say?
What the GPL says is all that is important. MySQL can try to reinterpret the GPL all they want, but they can't do it. They released MySQL under it, and they are therefore bound by it.
The GPL basically says that if you LINK to GPL software, and then distribute said software, you must GPL it. In the case of PHP, the linking occurs when you compile PHP against MySQL, not when you write PHP code.
Since the GPL only kicks in when you distribute, it means that should you distribute the PHP binaries with MySQL you would need to GPL PHP. But PHP has a license that explicitly disallows relicensing, so technically, you cannot distribute the PHP interpreter binaries and MySQL connect libs linked together under the GPL, only the commercially licensed MySQL would allow you to do this.
What you do with your PHP code is none of their damned business, as it is never linked to anything, it is PHP code. Since the PHP interpreter in binary form cannot be redistributed with GPLd MySQL, it's a non-issue. I.e. you can't create a system that would require you to GPL your PHP code, since PHP cannot be relicensed.
