Method to test if two values are equal. To use this rule, the form
XML needs a validate attribute of equals and a field attribute
that is equal to the field to test against.
Parameters
\SimpleXMLElement $element The SimpleXMLElement object representing the `<field>` tag for the form field object.
mixed$value The form field value to validate.
string$group The field name group control value. This acts as an array container for the field.
For example if the field has name="foo" and the group value is set to "bar" then the
full field name would end up being "bar[foo]".
\Joomla\Registry\Registry $input An optional Registry object with the entire data set to validate against the entire form.
\Joomla\CMS\Form\Form $form The form object for which the field is being tested.