Doubt - Generate 6 character random string in PHP

Question

Guys, how do I generate a string like this:

Z8Yjv.jpg

I went to get a photo of a user here from Stack-Overflow and the name of the photo came like this. I had thought of something like this:

<?php

$string = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

echo substr(str_shuffle($string), 0, 6);

?>

What is the probability of generating the same string??... And if it is wrong in this way, then what is the best way to generate it???