User Registration - Salt Help

[Mko]

So I'm developing a Plugin where upon user registration, there will be a new salt (only 5 digits long) and hash created and inserted into a table, and in the user table, there will be the default salt and hash length (of around 30).

Basically, I need to know what to put after $salt to make it generate a 5 digit salt which would consist of all the characters in the normal 30-digit salt (such as a-z, A-Z, 0-9, !, @, etc.).

Here's what I've tried:
$salt = chr(rand(33, 126));
^That one only generates 1 digit, which is not what I'm looking for.

$salt = substr(uniqid(rand(), true), 0, 5);
^That one generates 5 digits but only numbers, no letters or other characters.