Does PHP have an array_map_recursive function that gives my callback access to the array's keys?

No. However, you can add this functionality to your project via the following code:

<?php
function array_map_recursive(Array &$arr, $fnMap)
{
    $fnHelper = function (Array $arr, $fnMap, $fnHelper)
    {
        $result = array();
        foreach ($arr as $key => $value)
        {
            if (is_array($value))
            {
                $fnMap($key, $null = NULL, $isKeyOnly = TRUE);
                $result[$key] = $fnHelper($value, $fnMap, $fnHelper);
                continue;
            }
            $fnMap($key, $value, $isKeyOnly = FALSE);
            $result[$key] = $value;
        }
        return $result;
    };
    $arr = $fnHelper($arr, $fnMap, $fnHelper);
}

Note that array_walk_recursive provides similar functionality but falls short of our requirements due to the following caveat copy and pasted directly from php.net:

"You may notice that the key 'sweet' is never displayed. Any key that holds an array will not be passed to the function."

Example usage would look like:

<?php
// EXAMPLE CALLBACK
$urldecode = function (&$key, &$value, $isKeyOnly) // NOTE THE 3 ARGS AND THE FACT THAT THE FIRST TWO ARE BY REF
{
    $key = urldecode($key);
    $value = urldecode($value);
};

// EXAMPLE DATA
$data = array(
    '%21' => '%21',
    '%40' => '%40',
    '%23' => array(
        '1' => '%23',
        '3' => '%23',
        '2' => '%23',
    ),
    '%24' => '%24',
    '%25' => '%25',
    '%5E' => '%5E',
    '%26' => '%26',
    '%2A' => '%2A',
);

// DEMO...
echo "BEFORE:\n";
print_r($data);
array_map_recursive($data, $urldecode);
echo "\nAFTER:\n";
print_r($data);

/*
THE EXAMPLE CODE ABOVE OUTPUTS:

BEFORE:
Array
(
    [%21] => %21
    [%40] => %40
    [%23] => Array
    (
        [1] => %23
        [3] => %23
        [2] => %23
    )
    [%24] => %24
    [%25] => %25
    [%5E] => %5E
    [%26] => %26
    [%2A] => %2A
)

AFTER:
Array
    (
    [!] => !
    [@] => @
    [#] => Array
    (
        [1] => #
        [3] => #
        [2] => #
    )
    [$] => $
    [%] => %
    [^] => ^
    [&] => &
    [*] => *
)
*/

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What's the fastest way to locate a WORD in a file via the command line?

cat path/to/your/file.txt | grep the_word_you_want_to_find

This technique comes in handy when you're trying to locate a certain configuration directive within a configuration file (as illustrated in the following example):

cat /etc/php5/apache2/php.ini | grep upload_max_filesize
upload_max_filesize = 2M

What's the fastest way to locate a FILE via the command line?

find / -name 'file.txt'

This technique comes in handy when you're trying to locate certain configuration files (as illustrated in the following example):

find / -name 'php.ini'
/Applications/MAMP/bin/php/php5.2.17/conf/php.ini
/Applications/MAMP/bin/php/php5.3.6/conf/php.ini

How can I tell which user Apache is running as using PHP?

Create a file in your root web directory named whoami.php with the following code inside:

<?php
echo shell_exec('whoami');

Then, access whoami.php using your browser at an address such as:

http://YOURdomain.com/whoami.php

The name of the user Apache is running as will be displayed in your browser.

When you're finished testing, delete whoami.php.