Generate an identifier code for invoices

generate_code.php

<?php
define('KEY_CHARS', 'ACEFGHJKPQRSTWXYZ23456789'); // characters which cannot be confused phonetically or by bad handwriting

function generate_code($len = 6) {
    $k = str_repeat('.', $len);
    while ($len--) {
        $k[$len] = substr(KEY_CHARS, mt_rand(0, strlen(KEY_CHARS) - 1), 1);
    }
    return $k;
}