Presumably there are only two page options, one for known customers and one for others?

Assuming customer numbers kept on a database and form using method='post'.

Your php code needs to do the following:

The code must start with; ob_start(); This buffers the header output until you have made up your mind what you want to do.

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$custnum = $_POST['custnum'];

... additional code

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$query = 'select id from customers where custnum ='.$custnum;

... more code

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if ($rows == 1){
   header('Location: http://domain.co.uk/customer.html');
}
else {
   header('Location: http://domain.co.uk/notcustomer.html');
}

... anything else you want to do

And finally:

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ob_end_flush();

Sends the header to the browser and the redirection takes place.