See here:
http://www.webcheatsheet.com/php/create_word_excel_csv_files_with_php.php
To quote from the article the most common method:
Using HTTP Headers
In this method you need to format the HTML/PHP page using
Word-friendly CSS and add header information to your PHP script. Make
sure you don't use external style sheets since everything should be in
the same file.
As a result user will be prompted to download a file. This file will
not be 100% "original" Word document, but it certainly will open in MS
Word application. You can use this method both for Unix and Windows
environments.
<?php
header("Content-type: application/vnd.ms-word");
header("Content-Disposition: attachment;Filename=document_name.doc");
echo "<html>";
echo "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=Windows-1252\">";
echo "<body>";
echo "<b>My first document</b>";
echo "</body>";
echo "</html>";
?>
