PHP Random string generation miraculously generated the same string

So I've got a fairly simple function in PHP that renders 10 character long order IDs:

function createReference($length = 10)
    {
        $characters = 'ABCDEFGHIJKLMNPQRSTUVWXYZ123456789';
        $string = '';
        for ($i = 0; $i < $length; $i++) {
            $string .= $characters[rand(0, strlen($characters) - 1)];
        }
        return $string;
    }

However, today on the 154020th table record, it generated the same 10-character ID as a previous order ID (which was the 144258th record in the table), and tried to insert it. Since I have a UNIQUE restriction on the column, I got an error and I received a notification from this.

According to my calculations, the script above creates 34^10 = 2.064.377.754.059.776 different possibilities.

I've read some stuff about rand() and mt_rand() doing different stuff but that shouldnt be an issue on PHP 7.1+. The script is running on PHP 7.3.

So should I buy a lottery ticket right now, or is there something predictable about the pseudo-randomness being used here? If so, what is a solution to have better distribution?