Short unique id in php

Make a small function that returns random letters for a given length:

<?php
function generate_random_letters($length) {
    $random = '';
    for ($i = 0; $i < $length; $i++) {
        $random .= chr(rand(ord('a'), ord('z')));
    }
    return $random;
}

Then you'll want to call that until it's unique, in pseudo-code depending on where you'd store that information:

do {
    $unique = generate_random_letters(6);
} while (is_in_table($unique));
add_to_table($unique);

You might also want to make sure the letters do not form a word in a dictionnary. May it be the whole english dictionnary or just a bad-word dictionnary to avoid things a customer would find of bad-taste.

EDIT: I would also add this only make sense if, as you intend to use it, it's not for a big amount of items because this could get pretty slow the more collisions you get (getting an ID already in the table). Of course, you'll want an indexed table and you'll want to tweak the number of letters in the ID to avoid collision. In this case, with 6 letters, you'd have 26^6 = 308915776 possible unique IDs (minus bad words) which should be enough for your need of 10000.

EDIT: If you want a combinations of letters and numbers you can use the following code:

$random .= rand(0, 1) ? rand(0, 9) : chr(rand(ord('a'), ord('z')));

There are two ways to obtain a reliably unique ID: Make it so long and variable that the chances of a collision are spectacularly small (as with a GUID) or store all generated IDs in a table for lookup (either in memory or in a DB or a file) to verify uniqueness upon generation.

If you're really asking how you can generate such a short key and guarantee its uniqueness without some kind of duplicate check, the answer is, you can't.

Really simple solution:

Make the unique ID with:

$id = 100;
base_convert($id, 10, 36);

Get the original value again:

intval($str,36);

Can't take credit for this as it's from another stack overflow page, but I thought the solution was so elegant and awesome that it was worth copying over to this thread for people referencing this.

You could use the Id and just convert it to base-36 number if you want to convert it back and forth. Can be used for any table with an integer id.

function toUId($baseId, $multiplier = 1) {
    return base_convert($baseId * $multiplier, 10, 36);
}
function fromUId($uid, $multiplier = 1) {
    return (int) base_convert($uid, 36, 10) / $multiplier;
}

echo toUId(10000, 11111);
1u5h0w
echo fromUId('1u5h0w', 11111);
10000

Smart people can probably figure it out with enough id examples. Dont let this obscurity replace security.

Letters are pretty, digits are ugly. You want random strings, but don't want "ugly" random strings?

Create a random number and print it in alpha-style (base-26), like the reservation "numbers" that airlines give.

There's no general-purpose base conversion functions built into PHP, as far as I know, so you'd need to code that bit yourself.

Another alternative: use uniqid() and get rid of the digits.

function strip_digits_from_string($string) {
    return preg_replace('/[0-9]/', '', $string);
}

Or replace them with letters:

function replace_digits_with_letters($string) {
    return strtr($string, '0123456789', 'abcdefghij');
}

10 chars:

substr(uniqid(),-10);

5 binary chars:

hex2bin( substr(uniqid(),-10) );

8 base64 chars:

base64_encode( hex2bin( substr(uniqid(),-10) ) );

function rand_str($len = 12, $type = '111', $add = null) {
    $rand = ($type[0] == '1'  ? 'abcdefghijklmnpqrstuvwxyz' : '') .
            ($type[1] == '1'  ? 'ABCDEFGHIJKLMNPQRSTUVWXYZ' : '') .
            ($type[2] == '1'  ? '123456789'                 : '') .
            (strlen($add) > 0 ? $add                        : '');

    if(empty($rand)) $rand = sha1( uniqid(mt_rand(), true) . uniqid( uniqid(mt_rand(), true), true) );

    return substr(str_shuffle( str_repeat($rand, 2) ), 0, $len);
}