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The best way to get this info is to use the class object handlers (in this case UserDataHandler. Once you use the handler you can get the userData object(s) and then perform a getVar on the udata_value.
The udata_value is stored as a serialized variable. The class does some 'sanity' checking when saving/retrieving the data to prevent errors. You could do the same checking as the UserData:;getVar() method does (make sure it's a serialized value, and unserialize it and then base64_decode the values) but it would be better to use the class methods "as intended".
You might look at the ./admin/reports.php to get an idea for how to set it up. Since you're doing this on the front side you'll want to look at ./index.php to make sure you include/use the classes you need to get everything setup.
If you post (or PM me) I can try and look over what you create but my time's limited for the next couple of weeks... If you post here maybe we can get others to 'chime in' and help along the way.
I'd start with:
use XmfFilterInput;
use XmfRequest;
use XoopsModulesXforms;
use XoopsModulesXformsConstants;
use XoopsModulesXformsUtility;
require __DIR__ . '/header.php';
$myts = MyTextSanitizer::getInstance();