Is www-data user the same as apache user on Centos system
Excerpt from :http://www.linuxquestions.org/questions/linux-newbie-8/is-www-data-some-sort-of-generic-user-933894/
On many (not all) distributions, www-data is the user under which the Apache web server runs. This also means that everything done by Apache (especially including PHP scripts) will be done with the permissions of user www-data (and also group www-data) by default.