How to implement a random float function so that it does not lose entropy? (PHP)

I am trying to generate random floats using nothing but bytes I get from /dev/urandom. Currently my best idea is to get the platform precision doing something like:

$maximumPrecision = strlen('' . 1/3) - 2;

and then construct a string of 0-9 in a loop the number of times $maximumPrecision tells us. For examle, if the precision is 12, I would generate 12 random numbers and concatenate them. I think it's an ugly idea.

Update: Does this make sense?

$bytes =getRandomBytes(7); // Just a function that returns random bytes.
$bytes[6] = $bytes[6] & chr(15); // Get rid off the other half
$bytes .= chr(0); // Add a null byte

$parts = unpack('V2', $bytes);

$number = $parts[1] + pow(2.0, 32) * $parts[2];
$number /= pow(2.0, 52);