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Processes new user registrations.
Checks the data provided by the user during signup. Verifies the validity and uniqueness of user names and user email addresses, and checks email addresses against admin-provided domain whitelists and blacklists.
<?php wpmu_validate_user_signup($user_name, $user_email); ?>
The hook 'wpmu_validate_user_signup' provides an easy way to modify the signup process. The value $result, which is passed to the hook, contains both the user-provided info and the error messages created by the function. 'wpmu_validate_user_signup' allows you to process the data in any way you'd like, and unset the relevant errors if necessary.
wpmu_validate_user_signup() is located in