$s = '0.'.mt_rand().mt_rand(100000, 999999);
for($i=0;$i<16;$i++) $s .= rand(0, 9);
$s = floatVal('0.'.mt_rand().mt_rand(100000, 999999)); //или $s = mt_rand().mt_rand(100000, 999999)/10000000000000000;