Not able to extract the link in node "<enc:enclosure rdf:resource…" from xml file with PHP

SimpleXML does some implicit namespace switching. You used the explicit syntax for the children already, you can do the same for the attributes.

However I suggest defining a constant/variable with all the namespaces you are using. This will make you code a lot more readable. The keys can be the different from the prefixes in the document.

$xmlns = [
    'enc' => 'http://purl.oclc.org/net/rss_2.0/enc/',
    'rdf' => 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
    // defined in the XML as namespace for elements without a prefix
    'rss' => 'http://purl.org/rss/1.0/',
];

$rdf = simplexml_load_string(getXMLString());

foreach ($rdf->children($xmlns['rss'])->item as $item) {
    foreach ($item->children($xmlns['enc'])->enclosure as $enclosure) {
        echo 'Length: ' . $enclosure->attributes($xmlns['enc'])['length'] . "n"; 
        echo 'Resource: ' . $enclosure->attributes($xmlns['rdf'])['resource'] . "n"; 
    } 
}

function getXMLString() {
    return <<<'XML'
<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:prism="http://prismstandard.org/namespaces/basic/2.0/" xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:enc="http://purl.oclc.org/net/rss_2.0/enc/" xmlns:cc="http://web.resource.org/cc/" xmlns="http://purl.org/rss/1.0/">
<item>
      <title><![CDATA[Canada moves to ban funding for ‘risky’ foreign collaborations]]></title>
      <link>https://www.science.org/content/article/canada-moves-ban-funding-risky-foreign-collaborations</link>
      <description><![CDATA[China is seen as main target in rejecting joint projects with certain foreign entities]]></description>
      <enc:enclosure rdf:resource="https://www.science.org/do/10.1126/science.adh2317/rss/_20230217_nid_canada_china.jpg" enc:length="165061" enc:type="image/jpeg" />
      <dc:title><![CDATA[Canada moves to ban funding for ‘risky’ foreign collaborations]]></dc:title>
      <dc:identifier>doi:10.1126/science.adh2317</dc:identifier>
      <dc:date>2023-02-17T05:55:00Z</dc:date>
      <dc:creator>Jeffrey Mervis</dc:creator>
      <prism:publicationName><![CDATA[Canada moves to ban funding for ‘risky’ foreign collaborations]]></prism:publicationName>
      <prism:coverDate>2023-02-17T05:55:00Z</prism:coverDate>
      <prism:coverDisplayDate>2023-02-17T05:55:00Z</prism:coverDisplayDate>
      <prism:doi>10.1126/science.adh2317</prism:doi>
      <prism:url>https://www.science.org/content/article/canada-moves-ban-funding-risky-foreign-collaborations</prism:url>
</item></rdf:RDF>
XML;
}

Output:

Length: 165061
Resource: https://www.science.org/do/10.1126/science.adh2317/rss/_20230217_nid_canada_china.jpg

DOM

DOM is more explicit and has a set of namespace aware methods with the suffix NS (for example getAttributeNS()).

DOMXpath::evaluate() allows for complex expressions to fetch nodes and scalar values:

$xmlns = [
    'enc' => 'http://purl.oclc.org/net/rss_2.0/enc/',
    'rdf' => 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
    // defined in the XML as namespace for elements without a prefix
    'rss' => 'http://purl.org/rss/1.0/',
];

$document = new DOMDocument;
$document->loadXML(getXMLString());
$xpath = new DOMXPath($document);
// register the namespaces 
foreach ($xmlns as $alias => $uri) {
    $xpath->registerNamespace($alias, $uri);
}
$items = [];
foreach($xpath->evaluate('//rss:item') as $itemNode) {
    $items[] = [
        // fetch "{http://purl.org/rss/1.0/}title" as string
        'title' => $xpath->evaluate('string(rss:title)', $itemNode),
        'enclosure' => [
            // fetch the attribute values
            'resource' => $xpath->evaluate('string(enc:enclosure/@rdf:resource)', $itemNode),
            'length' => $xpath->evaluate('number(enc:enclosure/@enc:length)', $itemNode)
        ],
    ];
}
var_dump($items);

Output:

array(1) {
  [0]=>
  array(2) {
    ["title"]=>
    string(66) "Canada moves to ban funding for ‘risky’ foreign collaborations"
    ["enclosure"]=>
    array(2) {
      ["resource"]=>
      string(85) "https://www.science.org/do/10.1126/science.adh2317/rss/_20230217_nid_canada_china.jpg"
      ["length"]=>
      float(165061)
    }
  }
}