PHP的唯一ID

Question

I write this simple line to get random & unique code each time (just 8 characters):

echo substr(md5(uniqid(rand(), true)),0,8);

Output:

077331e5
5af425b1
0fc7dcf2
...

I ask if I'll never get a collision (duplicate). Or that can happen.

BS:
It's better to use time() ?

echo substr(md5(uniqid(time(), true)),0,8);

Answer 1

Hashes can have collisions. By taking a substring of the hash you are just upping the chance of that happening.

Answer 2

Regardless of what you feed into md5(), by doing the substring, you're eliminating a large part of md5's output and constricting the range of possible hashes. md5 outputs a 128bit string, and you're limiting it to 32bits, So you've got from a 1 in 1x10^38 to 1 in 4 billion chance of a collision.

Answer 3

Your "unique code" is a string of eight hexadecimal digits, and thus it has 4294967296 possible values. You are thus guanteed to get a duplicate of an earlier code by the 4294967297th time you run it.

Answer 4

PHP has a method to provide unique Ids called uniqid()

You stand a fair chance of your 8 char MD5 being unique but as with any random string the shorter you make the more likely you are to have a collision.

Answer 5

Short answer: it can happen. There's a discussion here about the collision space of MD5 that you might want to check out. Doing a substring of the MD5 will make the collision space much, much larger.

A better solution may be the answer proposed here , possibly checking it against other unique IDs that you've generated.

Answer 6

Your code returns part of a hash. Hashes are for hashing, thus you can not guarantee any pattern within the results (eg. uniqueness).

Also, you are getting only part of a hash, and each letter from a hash is hexadecimal (from 0 to 9 or from a to b - 16 possibilities). It needs only a simple calculation:

16 ^ 8 = 4 294 967 296

to find how many unique values can your code generate. This number ( 4 294 967 296 ) means, that if you use this function more thatn 4 294 967 296 times, the value generated with it surely will not be unique . Of course it is certain, that in this case the number of iterations will not be unique after applying it to smaller number of values.