二分查找算法
function bin_sch($array, $low, $high, $k){
	
   if ($low <= $high){
	$mid = intval(($low+$high)/2);
	
	if ($array[$mid] == $k){
	return $mid;
	}elseif ($k < $array[$mid]){
		return bin_sch($array, $low, $mid-1, $k);
	}else{
		return bin_sch($array, $mid+1, $high, $k);
	}
   }
	return -1;
}