Simple ascii random string generator

generate_random_string.php

<?php 

function generateRandomString($length = 8)
{
    if (0 >= $length || !is_int($length)) {
        $msg = sprintf('Exception! "%s()" only accepts positive integer values of 1 or more', __FUNCTION__);
        throw new UnexpectedValueException($msg);
    }

    $ascii = array_merge(
        range(48, 57), 
        range(65, 90), 
        range(97, 122)
    );

    $random = array();
    $limit  = count($ascii) - 1; 
     
    while (0 !== ($length--)) {
        $n = mt_rand(0, $limit);
        $random[] = chr($ascii[$n]);
    }

    return implode($random);
}

// generate!

system('clear');

define('RAND_LENGTH', 8);

try {
    echo generateRandomString(RAND_LENGTH) . PHP_EOL;
} catch (UnexpectedValueException $e){
    echo $e->getMessage() . PHP_EOL;
    exit();
}