How to generate alphanumeric OTP in PHP

<?php
/* This function will return a random */
/* string of specified length */
function random_strings($length_of_string)
{
 
    /* String of all alphanumeric character */
    $str_result = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
 
    /* Shufle the $str_result and returns substring */
    /* of specified length */
    return substr(str_shuffle($str_result),  
                       0, $length_of_string);
}
echo random_strings(6);