In a nutshell – you can’t. PHP code is by nature plain non-compiled text and if someone really wants to copy it they shall.
On top of that licensing considerations often strip whatever small things you can try. If you choose to or have to release under GPL you cannot forbid redistribution or put in technical means for that either.
One practical solution is too restrict access to updates. This doesn’t solve redistribution, but this does make sure only paid users are getting latest versions immediately and efficiently. This model is successfully used for some high profile WordPress extensions, for example BackupBuddy