二分 BinarySearch

function BinarySearch($arr, $low, $top, $value)
{

  // low 和 top 可能重叠
  while ($low <= $top)
  {
    $mid = floor( ($low+$top)/2 );

    if ($arr[$mid] == $value) 
    {
      return $mid;
    }

    if ($arr[$mid] < $value)
    {
      $low = $mid + 1;
    }
    else
    {
      $top = $mid - 1;
    }

  }
}

$arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];

echo BinarySearch($arr, 0, count($arr)-1, 6);