PHP Xpath : get all href values that contain needle

Not sure I understand the question correctly, but the second XPath expression already does what you are describing. It does not match against the text node of the A element, but the href attribute:

$html = <<< HTML
<ul>
    <li>
        <a href="">Description</a>
    </li>
    <li>
        <a href="">Description</a>
    </li>
</ul>
HTML;

$xml  = simplexml_load_string($html);
$list = $xml->xpath("//a[contains(@href,'foo')]");

Outputs:

array(1) {
  [0]=>
  object(SimpleXMLElement)#2 (2) {
    ["@attributes"]=>
    array(1) {
      ["href"]=>
      string(31) ""
    }
    [0]=>
    string(11) "Description"
  }
}

As you can see, the returned NodeList contains only the A element with href containing foo (which I understand is what you are looking for). It contans the entire element, because the XPath translates to Fetch all A elements with href attribute containing foo. You would then access the attribute with

echo $list[0]['href'] // gives ""

If you only want to return the attribute itself, you'd have to do

//a[contains(@href,'foo')]/@href

Note that in SimpleXml, this would return a SimpleXml element though:

array(1) {
  [0]=>
  object(SimpleXMLElement)#3 (1) {
    ["@attributes"]=>
    array(1) {
      ["href"]=>
      string(31) ""
    }
  }
}

but you can output the URL now by

echo $list[0] // gives ""