title: Convert random values from /dev/urandom
body: <p>As mentioned in the <a href="http://www.php.net/manual/en/function.mt-rand.php#105587" rel="noreferrer">comments of <code>mt_rand()</code></a> it is weak in security and we should use /dev/urandom instead. My problem is that from urandom I get a binary string.</p>
  
  <p>How do I convert this binary string to 0-9a-zA-Z?</p>
  
  <p>Looks like <code>base_convert()</code> does not work here.</p>