include_once($phpbb_root_path . 'includes/functions_profile_fields.' . $phpEx);
$user_id = 2; //The id of the user that you want to get the custom profiles fields
$cp = new custom_profile();
$user_fields = $cp->generate_profile_fields_template('grab', $user_id);
This creates a multidimensional array in $user_fields containing all the selected users custom fields, including the value and all the field data.