Enable Write Permissions for the Native Built-In Apache in Mac OS X Lion
When using the native built in version of Apache in Lion OS X you won't have write permissions unless you change the User and Group settings in apache's httpd.conf file. Here are some quick instructions on how to achieve that.
Open Terminal and Enter
sudo nano /private/etc/apache2/httpd.conf
Find and change http.conf code from
User _www Group _www
User your_mac_username Group staff # With earlier versions such as Leopard, capitalise staff to Staff
Restart apache by going to System Preferences > Sharing and un-tick then tick Web Sharing.
PHP to check write permissions are enabled
echo (is_writable('.')) ? "writable" : "not writable";