Why is XPath count() not working for me in PHP?

I am trying to get the amount of all section heading elements (H1,H2,H3,H4,H5,H6) from a document using XPath, e.g.:

    $headerCount=$xpath->query("count(//*[self::h1 or self::h2 or self::h3 or self::h4 or self::h5 or self::h6])"); 

I tested above query on http://www.xpathtester.com/xpath with this example and got '6' as result:

<h2>Header 1</h2>
<h2>Header 2</h2>
<h3>Header 2.1</h3>
<h4>Header 2.1.1</h4>
<h2>Header 3</h2>
<h3>Header 3.1</h3>

But when I try to use $headerCount like this:

echo "count: ".$headerCount->textContent;

I get this error:

Undefined property: DOMNodeList::$textContent

DOMNodeList suggests that there is a list of nodes, but I thought it is just one numeric value, such as 6 for the amount of all headers.

Anybody who could shed some light on how I can just get the amount of all headings in a document?

Your XPath is fine; it's your PHP that requires adjustment (as Alejandro pointed out in a helpful comment).

Use DOMXPath::evaluate instead of DOMXPath::query:

$headerCount=$xpath->evaluate("count(//*[self::h1 or self::h2 or self::h3 or self::h4 or self::h5 or self::h6])"); 

Then $headerCount will be just one numeric value, such as 6 for the amount of all headers, as expected.