required_if Yes Returns FALSE if no value is provided and another input has a specific value
  1. Name of the other input to check for
  2. Value of the other input
      3. If you want to specify human readable values, you can add "^text" to one or both of the options
      eg: required_if[required_input^display_text]
      You can use language directives instead if you want …
      eg: required_if[required_input^lang:language_directive]
      NB If the first element is not in $_POST, the rule will return TRUE. 
          required_if
      [other_field_name^lang:other_field_prompt
      ,other_field_value^lang:other_field_value_name
      ]