precision test

ini_set('precision', 64);
$a=(float)12345678.0;
$b=(float)12345679.0;
$c=(float)1.01233995;
$p=($a+$b+$c)/2;
$result=($p-$b)*$p;
var_dump($result);
$result=(($a+$b+$c)/2.0-$b)*$b;
var_dump($result);

$a='12345678.0';
$b='12345679.0';
$c='1.01233995';
$precision = 64;
$p=
bcdiv(
  bcadd(
    bcadd($a,$b,$precision)
  ,$c,$precision)
,'2',$precision);
$result=
bcmul(
  bcsub($p,$b,$precision)
,$p,$precision);
var_dump($result);
$result=
bcmul(
  bcsub(
    bcdiv(
      bcadd(
        bcadd($a,$b,$precision)
      ,$c,$precision)
    ,'2',$precision)
  ,$b,$precision)
,$b,$precision);
var_dump($result);

//float(76172.52840540916076861321926116943359375)
//float(76172.528367340564727783203125)
//string(70) "76172.5308260935915006250000000000000000000000000000000000000000000000"
//string(70) "76172.5307880250000000000000000000000000000000000000000000000000000000"