Centos 7 / Apache / PHP - mkdir(): Permission denied

Could be that although you have 755/777 permissions, SELinux is blocking httpd from writing/creating dirs.

Try:

chcon -R -t httpd_sys_content_t /path/to/www
chcon -R -t httpd_sys_content_rw_t /path/to/www/dir/for/rw

Further info: http://wiki.centos.org/TipsAndTricks/SelinuxBooleans