Random string in PHP7

I want to use PHP7's shiny new random_bytes() function to create a 8 and a 12 random character string.

In the official PHP doc there's just an example how to create a hexadecimal string by using bin2hex(). To have greater randomness I would like to generate an alphanumeric [a-zA-Z0-9] string, but couldn't find a way how to achieve this.

Thanks in advance for your help
ninsky