Open('doc1.docx');
$content1 = $zip->FileRead('word/document.xml');
$zip->Close();

// Extract the content of the first document
$p = strpos($content1, ' not found in document 1.");
$p = strpos($content1, '>', $p);
$content1 = substr($content1, $p+1);
$p = strpos($content1, '');
if ($p===false) exit("Tag  not found in document 1.");
$content1 = substr($content1, 0, $p);

// Insert into the second document
$zip->Open('doc2.docx');
$content2 = $zip->FileRead('word/document.xml');
$p = strpos($content2, '');
if ($p===false) exit("Tag  not found in document 2.");
$content2 = substr_replace($content2, $content1, $p, 0);
$zip->FileReplace('word/document.xml', $content2, TBSZIP_STRING);

// Save the merge into a third file
$zip->Flush(TBSZIP_FILE, 'merge.docx');