6 digit alphanumeric random code generator php

function generateRandomString($length = 25) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    return $randomString;
}
//usage 
$myRandomString = generateRandomString(5);<?php 
// online code for creating alphanumeric in php 
// this will generate 6 charactor, you can create as many just change the 6 from code
$pass = substr(str_shuffle("0123456789abcdefghijklmnopqrstvwxyz"), 0, 6);
echo $pass;

//output : 17w2y8
?>