array_rand() array to string?

Random colors should be generated in simplier way:

printf('%02x%02x%02x',mt_rand(0,255),mt_rand(0,255),mt_rand(0,255));

or

printf('%06x',mt_rand(0,16777215));

If you need to save color to variable, use sprintf instead of printf

Since the items are all different, you can turn them into keys rather than values, then use array_rand on the result:

implode('', array_rand(array_flip($hex), 6));

However, there may be a better way of achieving your overall goal. For example, if the overall goal allows for repetitions of digits, simply generate a random number from 0 through 0xFFFFFF and convert to a hex string:

dechex(mt_rand(0, 0xFFFFFF));

1. Why it stores elements' positions instead of elements in the first place?

   From the manual page:

   This is done so that you can pick random keys as well as values out of the array.

2. why does array_rand() NEVER choose a letter as the first element, and almost never as the second/third (99% of generated colors look like 11111a 12345c 123456)?

   array_rand uses rand (php_rand, in the C source). Depending on your system, php_rand is rand, random or lrand48. rand is a particularly poor random number generator.

array_rand() returns the keys of the randomly picked elements (see manual, section Return Values).

In order for it to work as expected, use array_flip() to retrieve the keys:

$random_color = array_rand(array_flip($hex), 6);

As for the "strange" results where there are almost no letters first elements, IDEOne and my server seem to reproduce these findings. A local machine running in my office (still running Debian etch / PHP 5.2.9) seems to disagree and evenly distribute elements from $hex... Seems to be a PHP version thing?

If you don't need to maintain the order of the $hex array, you could substitute this with shuffle(). Something like this (codepad example):

<?php
$hex = array("0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f");
shuffle($hex);
echo implode(array_slice($hex, 0, 6));

If you just want a random string of hex digits, you could also do something like

substr(md5(time()),-6);

or

substr(md5(uniqid()),-6);

You would get similar results without having to mess with the array.

is there any way to store array elements within array_rand()? Why it stores elements' positions instead of elements in the first place?

According to array_rand documentation, it returns the array 'keys', not the 'values'. Since your array is not an associative array, the keys are numbers. You'd need to do (untested):

$result = "";
$random_color = array_rand($hex, 6);
foreach ($random_color as $randomIndex) {
  $result = $result . $hex[$randomIndex];
}

Don't use array shuffle or array_rand because elements cannot repeat with that approach. That's not what you are trying to do.

what's the best way to do what I want to achieve?

If you want to generate a random color, you can use:

$color = '';
while(strlen($c) < 6) {
    $color .= sprintf("%02X", mt_rand(0, 255));
}

why does array_rand() NEVER choose a letter as the first element, and almost never as the second/third (99% of generated colors look like 11111a 12345c 123456)?

You may need to initialize the random numbers generator, but this is just a guess (see Timur's comment to this answer).

mt_srand((double)microtime()*1000000);