Comment on WordPress Comment Form Title by SEO Dave.
After a Google search or two looks like the private declaration doesn’t work with PHP4.
If you are running PHP4 try changing the word “private” to “var” on lines 54 and 55.
Not tested this.
Regarding PHP4: WordPress is moving away from supporting this old version of PHP, so you should look to upgrading to PHP 5 in the near future if this was the problem.