Random string generator

coupon_generator.php

<?php

/*
 * Coupon code generator
 */

$num_of_coupons_to_generate = 100;
$coupon_count = 0;

while($coupon_count != $num_of_coupons_to_generate)
{
    echo generate_coupon() . PHP_EOL;
    $coupon_count++;
}

function generate_coupon()
{
    $alph = 'ABCDEFGHJKMNPQRSTUVWXY';
    $num = '23456789';

    return 'V2-' .
        get_random_string($alph, 2) .
        get_random_string($num, 2) .
        get_random_string($alph, 2);
}

function get_random_string($valid_chars, $length)
{
    // start with an empty random string
    $random_string = "";

    // count the number of chars in the valid chars string so we know how many choices we have
    $num_valid_chars = strlen($valid_chars);

    // repeat the steps until we've created a string of the right length
    for ($i = 0; $i < $length; $i++)
    {
        // pick a random number from 1 up to the number of valid chars
        $random_pick = mt_rand(1, $num_valid_chars);

        // take the random character out of the string of valid chars
        // subtract 1 from $random_pick because strings are indexed starting at 0, and we started picking at 1
        $random_char = $valid_chars[$random_pick-1];

        // add the randomly-chosen char onto the end of our string so far
        $random_string .= $random_char;
    }

    // return our finished random string
    return $random_string;
}