The core PHP developer decided to deprecated the PHP 5.2.x branch more than five years ago (06 Jan 2011) [1] and as you can see here [2] the number of WordPress installations using PHP 5.2 is very low compared with the others.
The Sucuri WordPress plugin introduced new features with version 1.7.18 that require an up to date PHP installation to take advantage of the performance improvements. The error that you are getting is specifically related with the accessing of a constant __DIR__ that was introduced with PHP 5.3. I don't know why you are still using PHP 5.2 and to be honest I would prefer to not support that version anymore, but this is a simple error so I fixed it just this time with commit a7bb95b [3].
Thanks for the report, and please consider to install a newer version of PHP.
[1] https://secure.php.net/eol.php
[2] https://wordpress.org/about/stats/
[3] https://github.com/Sucuri/sucuri-wordpress-plugin/pull/17/commits/a7bb95b