how about this simple solution:
abs(1-mt_rand()/mt_rand())
or
/**
* Generate Float Random Number
*
* @param float $Min Minimal value
* @param float $Max Maximal value
* @param int $round The optional number of decimal digits to round to. default 0 means not round
* @return float Random float value
*/
function float_rand($Min, $Max, $round=0){
//validate input
if ($min>$Max) { $min=$Max; $max=$Min; }
else { $min=$Min; $max=$Max; }
$randomfloat = $min + mt_rand() / mt_getrandmax() * ($max - $min);
if($round>0)
$randomfloat = round($randomfloat,$round);
return $randomfloat;
}