Bug 2181 is marked closed, but last comment indicates it should be re-opened (sorry, I'm not the opener of 2181).  This would be what $struct->type would normally be used for:

    $types = array(
      0 => "TEXT",
      1 => "MULTIPART",  
      2 => "MESSAGE",
      3 => "APPLICATION",
      4 => "AUDIO",
      5 => "IMAGE",
      6 => "VIDEO",
      7 => "OTHER"
    );
    if($struct->type) {
      $type = $types[$struct->type];
    } else {
      $type = "UNKNOWN";
    }

$struct->type will always evaluate as false, but you can see from serializing the struct object that the type isn't being set at all.  This probably should be fixed either way.

On the other hand, can we always assume that if $struct->type is false/null/whatever, it is a text part?