randomstring.php

<?php

function randomString($length, $characterOptions = 'A0') {
    $characterClasses = array(
        'A' => 'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
        'a' => 'abcdefghijklmnopqrstuvwxyz',
        '0' => '0123456789',
        '!' => '~!@#$^&*_+-'
    );

    if (strlen($characterOptions) > count($characterClasses)) {
        $characters = $characterOptions;
    } else {
        $characterOptionsArray = str_split($characterOptions, 1);
        $characters = '';

        foreach ($characterOptionsArray as $char) {
            if (array_key_exists($char, $characterClasses)) {
                $characters .= $characterClasses[$char];
            }
        }
    }

    $charLen = strlen($characters);
    $randomChars = [];
    $cryptoStrong = false;

    if (function_exists('openssl_random_pseudo_bytes')) {
        $randomChars = unpack('C*', openssl_random_pseudo_bytes($length, $cryptoStrong));
    } elseif (function_exists('mcrypt_create_iv')) {
        $randomChars = unpack('C*', mcrypt_create_iv($length));
        $cryptoStrong = true;
    } else {
        for ($i = 0; $i < $length; $i++) {
            $randomChars[] = mt_rand();
        }
    }

    $string = '';
    foreach ($randomChars as $c) {
        $offset = (int)$c % $charLen;
        $string .= substr($characters, $offset, 1);
    }

    return $string;
}