PHP SimpleXML选择元素

I'm trying to extract information from an ncx file via SimpleXML. The file looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<ncx xmlns="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">
<head>
    <meta name="dtb:uid" content="http://www.hxa7241.org/articles/content/epup-guide_hxa7241_2007_1.epub"/>
</head>
<docTitle>
    <text>Der Weg der Könige</text>
</docTitle>
<navMap>
    <navPoint id="toc1" playOrder="1">
        <navLabel>
            <text>Widmung</text>
        </navLabel>
        <content src="e9783641059446_ded01.html"/>
    </navPoint>
    <navPoint id="toc2" playOrder="2">
        <navLabel>
            <text>Inhaltsverzeichnis</text>
        </navLabel>
        <content src="e9783641059446_toc01.html"/>
    </navPoint>
    <navPoint id="toc3" playOrder="3">
        <navLabel>
            <text>PRÄLUDIUM</text>
        </navLabel>
        <content src="e9783641059446_fm02.html"/>
    </navPoint>
    <navPoint id="toc4" playOrder="4">
        <navLabel>
            <text>4500 JAHRE SPÄTER</text>
        </navLabel>
        <content src="e9783641059446_fm03.html"/>
    </navPoint>
    <navPoint id="toc5" playOrder="5">
        <navLabel>
            <text>PROLOG - TÖTEN</text>
        </navLabel>
        <content src="e9783641059446_fm04.html"/>
    </navPoint>
    <navPoint id="toc6" playOrder="6">
        <navLabel>
            <text>ERSTER TEIL - Über dem Schweigen</text>
        </navLabel>
        <content src="e9783641059446_p01.html"/>
        <navPoint id="toc7" playOrder="7">
            <navLabel>
                <text>1 - STURMGESEGNET</text>
            </navLabel>
            <content src="e9783641059446_c01.html"/>
        </navPoint>
        <navPoint id="toc8" playOrder="8">
            <navLabel>
                <text>2 - DIE EHRE IST TOT</text>
            </navLabel>
            <content src="e9783641059446_c02.html"/>
        </navPoint>
        <navPoint id="toc9" playOrder="9">
            <navLabel>
                <text>3 - DIE STADT DER GLOCKEN</text>
            </navLabel>
            <content src="e9783641059446_c03.html"/>
        </navPoint>
        <navPoint id="toc10" playOrder="10">
            <navLabel>
                <text>4 - DIE ZERBROCHENE. EBENE</text>
            </navLabel>
            <content src="e9783641059446_c04.html"/>
        </navPoint>
        <navPoint id="toc11" playOrder="11">
            <navLabel>
                <text>5 - HÄRETISCH</text>
            </navLabel>
            <content src="e9783641059446_c05.html"/>
        </navPoint>
        <navPoint id="toc12" playOrder="12">
            <navLabel>
                <text>6 - BRÜCKE VIER</text>
            </navLabel>
            <content src="e9783641059446_c06.html"/>
        </navPoint>
        <navPoint id="toc13" playOrder="13">
            <navLabel>
                <text>7 - ALLES, WAS VERNÜNFTIG IST</text>
            </navLabel>
            <content src="e9783641059446_c07.html"/>
        </navPoint>
        <navPoint id="toc14" playOrder="14">
            <navLabel>
                <text>8 - NÄHER ZUR FLAMME</text>
            </navLabel>
            <content src="e9783641059446_c08.html"/>
        </navPoint>
        <navPoint id="toc15" playOrder="15">
            <navLabel>
                <text>9 - VERDAMMNIS</text>
            </navLabel>
            <content src="e9783641059446_c09.html"/>
        </navPoint>
        <navPoint id="toc16" playOrder="16">
            <navLabel>
                <text>10 - GESCHICHTEN ÜBER CHIRURGEN</text>
            </navLabel>
            <content src="e9783641059446_c10.html"/>
        </navPoint>
        <navPoint id="toc17" playOrder="17">
            <navLabel>
                <text>11 - TROPFEN</text>
            </navLabel>
            <content src="e9783641059446_c11.html"/>
        </navPoint>
    </navPoint>
    <navPoint id="toc18" playOrder="18">
        <navLabel>
            <text>ZWISCHENSPIELE</text>
        </navLabel>
        <content src="e9783641059446_p02.html"/>
        <navPoint id="toc19" playOrder="19">
            <navLabel>
                <text>Z-1 - ISCHIKK</text>
            </navLabel>
            <content src="e9783641059446_c12.html"/>
        </navPoint>
        <navPoint id="toc20" playOrder="20">
            <navLabel>
                <text>Z-2 - NAN BALAT</text>
            </navLabel>
            <content src="e9783641059446_c13.html"/>
        </navPoint>
        <navPoint id="toc21" playOrder="21">
            <navLabel>
                <text>Z-3 - DER SEGEN DER UNWISSENHEIT</text>
            </navLabel>
            <content src="e9783641059446_c14.html"/>
        </navPoint>
    </navPoint>
    <navPoint id="toc22" playOrder="22">
        <navLabel>
            <text>ZWEITER TEIL - Die leuchtenden Stürme</text>
        </navLabel>
        <content src="e9783641059446_p03.html"/>
        <navPoint id="toc23" playOrder="23">
            <navLabel>
                <text>12 - EINHEIT</text>
            </navLabel>
            <content src="e9783641059446_c15.html"/>
        </navPoint>
        <navPoint id="toc24" playOrder="24">
            <navLabel>
                <text>13 - ZEHN HERZSCHLÄGE</text>
            </navLabel>
            <content src="e9783641059446_c16.html"/>
        </navPoint>
        <navPoint id="toc25" playOrder="25">
            <navLabel>
                <text>14 - ZAHLTAG</text>
            </navLabel>
            <content src="e9783641059446_c17.html"/>
        </navPoint>
        <navPoint id="toc26" playOrder="26">
            <navLabel>
                <text>15 - DER KÖDER</text>
            </navLabel>
            <content src="e9783641059446_c18.html"/>
        </navPoint>
        <navPoint id="toc27" playOrder="27">
            <navLabel>
                <text>16 - KOKONS</text>
            </navLabel>
            <content src="e9783641059446_c19.html"/>
        </navPoint>
        <navPoint id="toc28" playOrder="28">
            <navLabel>
                <text>17 - EIN BLUTROTER SONNENUNTERGANG</text>
            </navLabel>
            <content src="e9783641059446_c20.html"/>
        </navPoint>
        <navPoint id="toc29" playOrder="29">
            <navLabel>
                <text>18 - DER GROSSPRINZ DES KRIEGES</text>
            </navLabel>
            <content src="e9783641059446_c21.html"/>
        </navPoint>
        <navPoint id="toc30" playOrder="30">
            <navLabel>
                <text>19 - DER STURZ DER STERNE</text>
            </navLabel>
            <content src="e9783641059446_c22.html"/>
        </navPoint>
        <navPoint id="toc31" playOrder="31">
            <navLabel>
                <text>20 - SCHARLACHROT</text>
            </navLabel>
            <content src="e9783641059446_c23.html"/>
        </navPoint>
        <navPoint id="toc32" playOrder="32">
            <navLabel>
                <text>21 - WARUM MENSCHEN LÜGEN</text>
            </navLabel>
            <content src="e9783641059446_c24.html"/>
        </navPoint>
        <navPoint id="toc33" playOrder="33">
            <navLabel>
                <text>22 - AUGEN, HÄNDE ODER KUGELN?</text>
            </navLabel>
            <content src="e9783641059446_c25.html"/>
        </navPoint>
        <navPoint id="toc34" playOrder="34">
            <navLabel>
                <text>23 - VIELSEITIG</text>
            </navLabel>
            <content src="e9783641059446_c26.html"/>
        </navPoint>
        <navPoint id="toc35" playOrder="35">
            <navLabel>
                <text>24 - DIE GALERIE DER LANDKARTEN</text>
            </navLabel>
            <content src="e9783641059446_c27.html"/>
        </navPoint>
        <navPoint id="toc36" playOrder="36">
            <navLabel>
                <text>25 - DER SCHLÄCHTER</text>
            </navLabel>
            <content src="e9783641059446_c28.html"/>
        </navPoint>
        <navPoint id="toc37" playOrder="37">
            <navLabel>
                <text>26 - STILLE</text>
            </navLabel>
            <content src="e9783641059446_c29.html"/>
        </navPoint>
        <navPoint id="toc38" playOrder="38">
            <navLabel>
                <text>27 - KLUFTDIENST</text>
            </navLabel>
            <content src="e9783641059446_c30.html"/>
        </navPoint>
        <navPoint id="toc39" playOrder="39">
            <navLabel>
                <text>28 - ENTSCHEIDUNG</text>
            </navLabel>
            <content src="e9783641059446_c31.html"/>
        </navPoint>
    </navPoint>
    <navPoint id="toc40" playOrder="40">
        <navLabel>
            <text>ZWISCHENSPIELE</text>
        </navLabel>
        <content src="e9783641059446_p04.html"/>
        <navPoint id="toc41" playOrder="41">
            <navLabel>
                <text>Z-4 - RYSN</text>
            </navLabel>
            <content src="e9783641059446_c32.html"/>
        </navPoint>
        <navPoint id="toc42" playOrder="42">
            <navLabel>
                <text>Z-5 - DER SAMMLER AXIES</text>
            </navLabel>
            <content src="e9783641059446_c33.html"/>
        </navPoint>
        <navPoint id="toc43" playOrder="43">
            <navLabel>
                <text>Z-6 - EIN KUNSTWERK</text>
            </navLabel>
            <content src="e9783641059446_c34.html"/>
        </navPoint>
    </navPoint>
    <navPoint id="toc44" playOrder="44">
        <navLabel>
            <text>DRITTER TEIL - Sterben</text>
        </navLabel>
        <content src="e9783641059446_p05.html"/>
        <navPoint id="toc45" playOrder="45">
            <navLabel>
                <text>29 - IRRMASSUNG</text>
            </navLabel>
            <content src="e9783641059446_c35.html"/>
        </navPoint>
        <navPoint id="toc46" playOrder="46">
            <navLabel>
                <text>30 - UNSICHTBARE FINSTERNIS</text>
            </navLabel>
            <content src="e9783641059446_c36.html"/>
        </navPoint>
        <navPoint id="toc47" playOrder="47">
            <navLabel>
                <text>31 - UNTER DER HAUT</text>
            </navLabel>
            <content src="e9783641059446_c37.html"/>
        </navPoint>
        <navPoint id="toc48" playOrder="48">
            <navLabel>
                <text>32 - SEITENTRAGEN</text>
            </navLabel>
            <content src="e9783641059446_c38.html"/>
        </navPoint>
        <navPoint id="toc49" playOrder="49">
            <navLabel>
                <text>33 - CYMATIK</text>
            </navLabel>
            <content src="e9783641059446_c39.html"/>
        </navPoint>
        <navPoint id="toc50" playOrder="50">
            <navLabel>
                <text>34 - STURMWAND</text>
            </navLabel>
            <content src="e9783641059446_c40.html"/>
        </navPoint>
        <navPoint id="toc51" playOrder="51">
            <navLabel>
                <text>35 - EIN LICHT ZU SEHEN</text>
            </navLabel>
            <content src="e9783641059446_c41.html"/>
        </navPoint>
        <navPoint id="toc52" playOrder="52">
            <navLabel>
                <text>36 - DIE LEKTION</text>
            </navLabel>
            <content src="e9783641059446_c42.html"/>
        </navPoint>
    </navPoint>
    <navPoint id="toc53" playOrder="53">
        <navLabel>
            <text>SCHLUSSBEMERKUNG</text>
        </navLabel>
        <content src="e9783641059446_bm01.html"/>
    </navPoint>
    <navPoint id="toc54" playOrder="54">
        <navLabel>
            <text>ARS ARCANUM</text>
        </navLabel>
        <content src="e9783641059446_bm02.html"/>
    </navPoint>
    <navPoint id="toc55" playOrder="55">
        <navLabel>
            <text>DANKSAGUNG</text>
        </navLabel>
        <content src="e9783641059446_ack01.html"/>
    </navPoint>
    <navPoint id="toc56" playOrder="56">
        <navLabel>
            <text>Die Sturmlicht-Chroniken werden fortgesetzt in:</text>
        </navLabel>
        <content src="e9783641059446_tea01.html"/>
    </navPoint>
    <navPoint id="toc57" playOrder="57">
        <navLabel>
            <text>Copyright</text>
        </navLabel>
        <content src="e9783641059446_cop01.html"/>
    </navPoint>
</navMap>
</ncx>

And I want to grab all the html files, which are located in this element:

<content src="e9783641059446_cop01.html"/>

Currently I'm trying it this way:

$ncx = simplexml_load_file($file);
$items = $ncx->navMap->children();
foreach ($items as $it) {
    echo $it->content['src'];
}

Problem is that the content nodes are not in the same depth level as you might have noticed. Does anyone know how to fix it?

The XML has a namespace. Try with

$ncx = simplexml_load_file('test.xml');
$ncx->registerXPathNamespace('x', 'http://www.daisy.org/z3986/2005/ncx/');
foreach ($ncx->xpath('//x:content/@src') as $src) {
    echo $src, PHP_EOL;
}

Without XPath:

$ncx = simplexml_load_file('test.xml', "SimpleXmlElement", 0, 'http://www.daisy.org/z3986/2005/ncx/', false);
foreach ($ncx->navMap->navPoint as $np) {
    echo $np->content->attributes()->src, PHP_EOL;
}

the content nodes are not in the same depth level

Also you can use DOMDocument class easily, to finding target tag. Note that getElementsByTagName() get tag by name and getAttribute() get value of attribute.

$dom = DOMDocument::load($file);
$contents = $dom->getElementsByTagName("content");
foreach($content as $content){
    echo $content->getAttribute("src");
}