Suppose I have a wordpress site live and it contains a custom form i.e to collect user's data for checkout of the product, but the data collected will be submitted to a given url (e.g., http://example.com) not on the wordpress site itself.
Is it possible? What to do?
Cross-domain POSTing, GETing is allowed.
So you can easily use form action as http://example.com.
Ex:
<form action="http://example.com">
<form action="http://example.com2" method="post">. Then you can access form data using $_POST['name'] in example2.com. Or have I misunderstand the question?
$_POST['name'] to get data in other domain. Even it on same domain you can't never trust user input values like $_POST['name'] or $_GET['name']. You have to validate them.