You can't use preOpen in PHP, as that JS method is client-side in the browser - the server only gets involved when the data is submitted by the user. Once submitted, there are events there that you can latch onto to, see the manual page for more info.
What use case did you have in mind?
One option might be to use preOpen on the client-side to send an Ajax request to the server, to tell it to do whatever it is you need to do.
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