If you came across this error while access webmail using horde then there is no need to panic as you are getting that error because of PHP SuPHP enabled for every PHP script which runs under apache.
To fix this create a horde.conf file at /etc/httpd/conf.d and add the below contents
<Directory /usr/share/psa-horde> suPHP_Engine off </Directory>
This will disable suphp engine for /usr/share/psa-horde directory and finally restart apache service for the changes to take effect.
