Trying to manipulate MS Word bookmarks using PHP and COM

I'm trying to manipulate some of the fields in a supplied Word 2003 document using native document bookmarks using PHP and COM, but I get an error that I have ever used the method. If I try to invoke the bookmarks directly to replace the text, I get the error: The range cannot be deleted.

 function testBkMrkDetails($word, $bookmarkName, $subsTxt) {

try {
   $BkMark = $word->ActiveDocument->Bookmarks($bookmarkName);
   $range = $BkMark->Range;
   if (!$range) {
      echo "Range not created ";
   }
   $range->Text = $subsTxt;
} catch (Exception $e) {
    echo "bookmark failed: " . $e->getMessage() . "\n";
}
}

      

        
        
        
      

    

I have searched Google using the error message and running for PHP and COM tutorials that seem to suggest that I am looking at FormFields, so I wrote this:

 function testFormFlds ($word, $bookmarkName, $subsTxt) {

try {
    $formField = $word->Selection->FormFields($bookmarkName);
    if (!$formField) {
        die("Form failed : " . $bookmarkName . " not found\n");
    }

    $formField->Result($subsTxt);
} catch (Exception $e){
    echo "FormField failed: " . $e->getMessage();
}

}

      

        
        
        
      

    

However, this suggests that the requested collection member does not exist, so I am assuming that I did not call the feild name correctly (it was from the doc). I would appreciate some guidance on how to address this and learn more about the underlying technology. Thanks, Iain