PHP: generate random string

random_string.php

/**
 * Generate random string
 *
 * Strong password definition on strongpasswordgenerator.com
 * - at least 15 characters
 * - uppercase and lowercase letters
 * - numbers
 * - symbols, such as ` ! " ? $ ? % ^ & * ( ) _ - + = { [ } ] : ; @ ' ~ # | \ < , > . ? /
 *
 * @param number $length
 * @param string $abc
 * @return string
 */
function generateRandomString($length = 20, $abc = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789`!"?$?%^&*()_-+={[}]:;@\'~#|\<,>.?/')
{
    return substr(str_shuffle(str_repeat($abc, $length)), 0, $length);
}