XSLT not applying when echoed from PHP

I don't understand why the xsl file is not being applied when the xml is echoed from php. I checked the output in postman and the data is also being recorded in the database but it is still not formatted in the browser. Am i missing something in php?

PHP File (addSong_xml.php)

<?php
    $title  = "";
    $artist = "";
    $year   = "";

    $songs = $_POST['txt_xml_songs'];
    $xml_Doc = new DOMDocument();
    $xml_Doc->loadXML($songs);

    if(!$xml_Doc->schemaValidate('songs.xsd'))
        echo "XML not valid.";
    else
    {
        include('db_connect.php');
        header('Content_Type: text/xml');

        $xml_songs = simplexml_import_dom($xml_Doc);

        foreach($xml_songs->Song as $song)
        {
            $title  = $song->Title;
            $artist = $song->Artist;
            $year   = $song->Year;

            if($title != "" && $title != null)
            {
                $sql = "SELECT * FROM Songs WHERE Title='$title'";
                $result = mysqli_query($mysqli, $sql);

                if(mysqli_num_rows($result) < 1)
                {
                    $sql ="INSERT INTO Songs (Title, Artist, Year) VALUES ('$title', '$artist', '$year')";

                    if(!mysqli_query($mysqli, $sql))
                    {
                        die('Error: ' . mysqli_error($mysqli));
                    }
                }
            }
        }

        $xml_output = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>";
        $xml_output = $xml_output."<?xml-stylesheet type=\"text/xsl\" href=\"songs.xsl\"?>";
        $xml_output = $xml_output.$songs;

        echo $xml_output;
        mysqli_close($mysqli);
    }

?>