Check SOAP array for object in PHP

I'm building a postcode lookup service that will autocomplete a form and resort to manual input if not found.

I have connected to the SOAP client, pulled in the data but now I need to check the array to see a value has been set and if not then do something else.

So far I have:

    $params = array('ah21'=>array('session_id'=>$sessionid,'input'=>"$postcode,$number",'data'=>'STREET_1,HOUSE_1,DISTRICT_LEVEL,ORGANISATION,DISTRICT_2,FORMATTED_PREMISE_NO,TOWN,POSTCODE,COUNTY,COUNTRY'));
    $address = $soapclient->__soapCall("ah21",$params);
    $address = $address->address;

I need to check if 'HOUSE_1' has a value and if not then show 'STREET_1' instead.

$firstline = NULL;

function checkaddress(){
global $firstline;
  if (isset($address->HOUSE_1 )){
            $firstline = $address->HOUSE_1;             
  } else {
    $firstline = $address->STREET_1;    
}

I'm getting nothing back? Also, if you haven't guessed I'm learning php so any pointers on my code would be a great help.

UPDATE: forgot to mention that I'm printing $firstline elsewhere so appear back into the form.