PHP Random Name instead of sequence - php

Question

I have a basic URL shortener script but I am having an issue. Each time I enter a new url to shorten the script just adds the next letter to the database (i.e. http://www.both.com/a then http://www.both.com/b then http://www.both.com/c etc). The problem is I don't want to people to be able to view those links by simply going in sequential order. How can I make it so each new url has a random 6 digit alpha and numerical name created? Also, I wanted to make the alpha both random upper and lower case.
<?php
require 'config.php';
header('Content-Type: text/plain;charset=UTF-8');
$url = isset($_GET['url']) ? urldecode(trim($_GET['url'])) : '';
if (in_array($url, array('', 'about:blank', 'undefined', 'http://localhost/'))) {
die('Enter a URL.');
}
if (strpos($url, SHORT_URL) === 0) {
die($url);
}
function nextLetter(&$str) {
$str = ('z' == $str ? 'a' : ++$str);
}
function getNextShortURL($s) {
$a = str_split($s);
$c = count($a);
if (preg_match('/^z*$/', $s)) { // string consists entirely of `z`
return str_repeat('a', $c + 1);
}
while ('z' == $a[--$c]) {
nextLetter($a[$c]);
}
nextLetter($a[$c]);
return implode($a);
}
$db = new mysqli(MYSQL_HOST, MYSQL_USER, MYSQL_PASSWORD, MYSQL_DATABASE);
$db->set_charset('utf8mb4');
$url = $db->real_escape_string($url);
$result = $db->query('SELECT slug FROM redirect WHERE url = "' . $url . '" LIMIT 1');
if ($result && $result->num_rows > 0) { // If there’s already a short URL for this URL
die(SHORT_URL . $result->fetch_object()->slug);
} else {
$result = $db->query('SELECT slug, url FROM redirect ORDER BY date DESC, slug DESC LIMIT 1');
if ($result && $result->num_rows > 0) {
$slug = getNextShortURL($result->fetch_object()->slug);
if ($db->query('INSERT INTO redirect (slug, url, date, hits) VALUES ("' . $slug . '", "' . $url . '", NOW(), 0)')) {
header('HTTP/1.1 201 Created');
echo SHORT_URL . $slug;
$db->query('OPTIMIZE TABLE `redirect`');
}
}
}
?>

Answer 1

try something like this :
function generateString($length) {
$alphabet = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$alphabetLength = strlen($alphabet);
$randomString = '';
for ($i = 0; $i < $length; $i++) {
$randomString .= $alphabet[rand(0, $alphabetLength - 1)];
}
return $randomString;
}
$short = generateRandomString(6);