When I create a new file or folder, what determines the group that owns it?

The new file or folder will be owned by the same group that owns the directory that you're adding it to (tested on Mac OS X).

How do I make it so all future files and dirs have the permissions I want (for example, 775 rather than 755 for new directories) on a linux machine?

On your linux machine, open the following file via the following command:

sudo vim /etc/pam.d/login

Then, add the following line to it:

session optional pam_umask.so umask=0002

Then, restart the machine.

How do I make it so all future files and dirs have the permissions I want (for example, 775 rather than 755 for new directories) on a Mac OS X machine?

On your Mac OS X machine, create or open the following file via the following command:

sudo vim /etc/launchd-user.conf

Then, copy and past the following text into it:

umask 002

Then, restart your computer.

What does PHP's print_r($_FILES) look like?

<?php
print_r($_FILES);
/*
THE ABOVE CODE OUTPUTS:
Array
(
    [image] => Array
        (
            [name] => aircraft war military fighter pilot cockpit planes vehicles jet aircraft 3000x1928 wallpaper_www.wallpaperhi.com_6.jpg
            [type] => image/jpeg
            [tmp_name] => /Applications/MAMP/tmp/php/phpzqf0Y6
            [error] => 0
            [size] => 383379
        )

)
*/

How do I increase PHP's resource limits?

Check/edit your php.ini file for the following settings:

; Maximum amount of memory a script may consume (8MB).
memory_limit = 256M

; Maximum size of POST data that PHP will accept.
post_max_size = 32M

; Maximum allowed size for uploaded files.
upload_max_filesize = 32M

What does PHP's getimagesize function return?

<?php
$getimagesize = getimagesize('example/image/path/file.png');
print_r($getimagesize);
exit;
/*
THE ABOVE CODE OUTPUTS:
Array
(
    [0] => 1748
    [1] => 1165
    [2] => 2
    [3] => width="1748" height="1165"
    [bits] => 8
    [channels] => 4
    [mime] => image/jpeg
)
*/

How can I add an existing user to an existing group via the command line on a Mac OS X machine?

dseditgroup -o edit -u your_admin_username -p -a the_username_of_user_you_want_to_add_to_group -t user the_name_of_the_group_you_want_to_add_user_to

If I wanted to add myself to the "www" group I would simply run the following command:

dseditgroup -o edit -u johnerck -p -a johnerck -t user www

Run the groups command to see that you've been added:

groups