Are readonly field protected server side?

$fields[] = new PMProRH_Field("company", "text",
                array("size"=>40,
                      "class"=>"company",
                      "profile"=>true,
                      "required"=>true,
                      "readonly"=>true));

If I make a field readonly using the readonly option above, does this merely give the field a ‘readonly’ attribute in the front-end html? Or does it actually also protect the field server-side from being posted to?

https://www.ads-software.com/plugins/pmpro-register-helper/