The API shows to call "State" after the method is called.
So this should be working code.
$archivals = new COM('Unreal.ArchivalCtrl') or die ('Unable to Launch Archival Com');
$archivals->ManualStart('localhost', 'test', true, false);
$status = $archivals->GetSourceStatus('localhost', 'test')->State();
echo "$status";
?>