This is something many newbies struggle with (been there).
Here's a quick reference on how to make a directory writable by Apache in Mac OS X without giving full permissions to everyone a.k.a chmod -R 777.
Set the ownership of your desired directory/file to the _www user:
sudo chown -R _www:staff path/to/folder
Set the permissions so that the _www can write in the directory/file without giving permissions to everyone else:
sudo chmod -R 755 path/to/folder
Happy coding :-)
Note: Works fine with built-in OSX apache, not tested with apps such as MAMP/XAMPP or Homebrew/MacPorts apache installs.