道格拉斯坐标抽稀算法【PHP实现】

$polyline = "116.621222,41.028553;116.621648,41.029848;.....";
$pl = split(";", $polyline);
$plstr = "";
$flag = 0;
$min = 0.0001;
for ($j=0; $j < count($pl)-1; $j=$j+3) { 
	$xy1 = split(",", $pl[$j]);
	$x1 = $xy1[0];
	$y1 = $xy1[1];
	$xy2 = split(",", $pl[$j+2]);
	$x2 = $xy2[0];
	$y2 = $xy2[1];
	$b = $x1 - $x2;
	$a = $y2 - $y1;
	$c = ($y1 - $y2)*$x1 - $y1*($x1 -$x2);
	$xys = split(",", $pl[$j+1]);
	$d = (abs($a * $xys[0] + $b * $xys[1] + $c)) / (sqrt($a * $a + $b * $b));
	if($d < $min) {
		if($flag == 0)
			$plstr .= $pl[$j+1];
		else
			$plstr .= ";".$pl[$j+1];
		$flag++;
	}
}
$polyline = $plstr;
//希望有技术大神多多提点，帮助我完善代码，万分感谢