สอบถาม การเพิ่มวันที่ทำการแล้วคำนวณ อัตโนมัติ โดยไม่นับวันหยุด เสาร์-อาทิตย์

<?php
$w_nm=array('Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat');
function targettime($cur_day, $add){
	$cur_time = strtotime($cur_day);
	$addday = $add + (floor(($add-1) / 5)*2);
	$target=strtotime('+'.$addday.' day', $cur_time);
	$dw=date('w', $target);
	if($dw=='0'||$dw=='6') $target=strtotime('+2 day', $target);
	return $target;
}
$cur_day = '2015-08-31';
for($i=1; $i<20; $i++){
	$rs=targettime($cur_day, $i);
	echo $w_nm[date('w', $rs)],' ', date('Y-m-d', $rs),'<br>';
}

function getExtraday($st, $en){
	global $db;
	$sql = "select count(dte) num_row from dateExtra where dte between '$st' and '$en' ";
	if($rs=$db->query($sql)) $ro=$rs->fetch_assoc(); else $ro['num_row']=0;
	return intval($ro['num_row']);
}
$db=new mysqli('localhost', 'test','test','test');
$cur_day = '2015-08-31';
$rs = targettime($cur_day, 20);
$ln=getExtraday($cur_day, date('Y-m-d', $rs));
for($i=0; $i<$ln; $i++){
   $rs += 24*60*60;
   while( ( $dw=intval(date('w', $rs)))==0 || $dw==6 ) $rs += 24*60*60;
   if(getExtraday($cur_day, date('Y-m-d', $rs)))  $rs += 24*60*60;
}
echo date('Y-m-d', $rs);