I‘m running the Mediawiki software under Apache web server running on the Unix operating systems. I’m getting the following in my php error log:

PHP Fatal error: Cannot redeclare wfProfileIn() (previously declared in /var/www/html/wiki/includes/profiler/Profiler.php:14) in /var/www/html/wiki/includes/ProfilerStub.php on line 25.

How do I fix this problem under Mediawiki version 1.18.0?

According to this:

StartProfiler.php isn’t a core file, it is created by the sysadmin if it is needed. A StartProfiler.sample is included, which can be copied to StartProfiler.php. Due to changes in 1.18, a StartProfiler.php from a previous version will be incompatible. StartProfiler.sample was updated, but StartProfiler.php couldn’t be updated because it doesn’t exist in the distribution.

StartProfiler.php is the core problem. Deleting it gets rid of that problem.

In other words just delete the StartProfiler.php file or rename the file as StartProfiler_old.php:
rm /var/www/html/wiki/includes/StartProfiler.php
OR
mv /var/www/html/wiki/includes/StartProfiler.php /var/www/html/wiki/includes/StartProfiler_OLD.php