Parse HTML files with PHP Handler to add login_check, dangerous?

Hi,

Recently, I've protected a bunch of HTML files by adding some PHP to it via the following rules in httpd.conf

<Directory /targetdirectory>
php_value auto_prepend_file /webdirectory/add.php
</Directory>

In add.php there is some code to check if a user is logged on. If not, then redirect to index.php.

I've heard that this solution is not a very good one, because of its performance cost and it being dangerous for it may, among other things, cause problems when processing multiple files at once due to a lack of locking or something.

My question is: can I use this solution safely?

Thanks in advance!