php readfile xml始终返回零大小

    if (isset($_REQUEST['action'])) {

$dobot = <<< 'dobotxml'
<root>
<row_StudioVersion>
<item_0>Ver-1412</item_0>
</row_StudioVersion>
</root>
dobotxml;

        $xml = new SimpleXMLElement($dobot);
        $arraysize = sizeof( $_POST ) - 1;
        $rowsize = 7;
        for ($i=0; $i < $arraysize/$rowsize; $i++ ) {
            $row = $xml->addChild("row{$i}");
            $row->addChild("item_0", $_POST["t_{$i}"]);
            $row->addChild("item_1", $_POST["n_{$i}"]);
            $row->addChild("item_2", $_POST["x_{$i}"]);
            $row->addChild("item_3", $_POST["y_{$i}"]);
            $row->addChild("item_4", $_POST["z_{$i}"]);
            $row->addChild("item_5", $_POST["r_{$i}"]);
            $row->addChild("item_10",$_POST["p_{$i}"]);
        }


        header('Content-disposition: attachment; filename="newfile.xml"');
        header('Content-type: "text/xml"; charset="utf8"');
        readfile("newfile.xml");
        exit;
    }

我确保$xml中有内容,如果我打印($xml),它将导出

SimpleXMLElement Object ( [row_StudioVersion] => SimpleXMLElement Object ( [item_0] => Ver-1412 ) [row0] => SimpleXMLElement Object ( [item_0] => 0 [item_1] => dsf [item_2] => 1230 [item_3] => 230 [item_4] => 043 [item_5] => 2130 [item_10] => 340 ) [row1] => SimpleXMLElement Object ( [item_0] => 2 [item_1] => 34 [item_2] => 0123 [item_3] => 430 [item_4] => 034 [item_5] => 023 [item_10] => 03 ) [row2] => SimpleXMLElement Object ( [item_0] => 0 [item_1] => SimpleXMLElement Object ( ) [item_2] => 0 [item_3] => 0 [item_4] => 0 [item_5] => 0 [item_10] => 0 ) )

预期产量

<?xml version="1.0"?>
<root>
    <row_StudioVersion>
        <item_0>Ver-1412</item_0>
    </row_StudioVersion>
    <row0>
        <item_0>0</item_0>
        <item_2>10</item_2>
        <item_3>20</item_3>
        <item_4>30</item_4>
        <item_5>0</item_5>
        <item_10>0</item_10>
    </row0>
    <row1>
        <item_0>0</item_0>
        <item_2>40</item_2>
        <item_3>50</item_3>
        <item_4>60</item_4>
        <item_5>0</item_5>
        <item_10>0</item_10>
    </row1>
</root>