将文件中的帖子保存到msword中

Question

i want to save a post from an html form into a msword file

<form method="post" action="process.php">
    <text name="comment"></text>
   <input type="submit" name="send" value="submit"/>
</form>

process.php

<?php
 if(isset($_POST['submit']))
   {
    $body=$_POST['comment'];
    $newfile = fopen("essay/last.docx", "w") ;
   $content = "$body";
   fwrite($newfile, $content);

  fclose($newfile);
}
?>

the created file is not a valid msword file. any help is appreciated