// The characters we want in the output
$chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
$count = strlen($chars);

// Generate 12 random bytes
$bytes = random_bytes(12);

// Construct the output string
$result = '';
// Split the string of random bytes into individual characters
foreach (str_split($bytes) as $byte) {
    // ord($byte) converts the character into an integer between 0 and 255
    // ord($byte) % $count wrap it around $chars
    $result .= $chars[ord($byte) % $count];
}

// That's all, folks!
echo($result."n");