How to create a UUID in php (without a external library)

Question

I'm using uniqid() in PHP to generate a UUID however, the returned string is something like: 4cca63b0d4be4

I thought UUIDs are suppose to be something like: 550e8400-e29b-41d4-a716-446655440000

How do you create a UUID in PHP?

Usually I use auto-increment in MySQL to get my unique IDs, however, sometimes I want to generate a UUID when a unique ID needs to be displayed to the user. I don't want users to get a ID for a ticket/reference for example of 1, 2, 3, etc, or even 10222. Not a good idea for users to know how many records there are in my database (to work out how many users I have etc). I'm not using a hash or hashing the UUID since that defeats the purpose of using a UUID, and i have no problems string the 128bit string. What are your comments on the chances of collisions? if i have a million records for example. Also reading on wiki i see there are 5 versions, v1 v2 v3 v4 v5... which one should i use?

Answer 1

uuid v4:

function gen_uuid() {
    return sprintf( '%04x%04x-%04x-%04x-%04x-%04x%04x%04x',
        // 32 bits for "time_low"
        mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff ),

        // 16 bits for "time_mid"
        mt_rand( 0, 0xffff ),

        // 16 bits for "time_hi_and_version",
        // four most significant bits holds version number 4
        mt_rand( 0, 0x0fff ) | 0x4000,

        // 16 bits, 8 bits for "clk_seq_hi_res",
        // 8 bits for "clk_seq_low",
        // two most significant bits holds zero and one for variant DCE1.1
        mt_rand( 0, 0x3fff ) | 0x8000,

        // 48 bits for "node"
        mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff ), mt_rand( 0, 0xffff )
    );
}

Taken from here.

Answer 2

While I cannot answer the question on how to create UUIDs in PHP, the question about collisions is an often-asked one on StackOverflow. See for example Are GUID collisions possible?.

In short, collisions are not mathematically impossible, but are sufficiently improbable to be as good as impossible.