OpenX is comming with its own PEAR installation.

If your hosting company has preinstalled PEAR for you you might got the error

PHP Fatal error:  Cannot redeclare _pear_call_destructors() (previously declared in /usr/share/php/PEAR.php:765) in /home/…/ads/openx-2.6.4/lib/pear/PEAR.php on line XXX

To solve this make .htaccess file in the webroot of OpenX  – dont put the .htaccess in your web root.  Here is the contents of .htaccess :

php_value include_path ".:/home/domains/guda/ads/a/lib/pear"

You must enable AllowOverride in your apache configuration

 <Directory /home/domains/guda/cenite.com/openx_webroot>
      AllowOverride all
  </Directory>

Ref: https://developer.openx.org/jira/browse/OX-1006

http://forum.openx.org/index.php?showtopic=503420381