PHP random string generator

I’m trying to create a randomized string in PHP, and I get absolutely no output with this:

<?php
    function RandomString()
    {
        $characters="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        $randstring = '';
        for ($i = 0; $i < 10; $i++) {
            $randstring = $characters[rand(0, strlen($characters))];
        }
        return $randstring;
    }

    RandomString();
    echo $randstring;

What am I doing wrong?