Find h3 and h4 tags beneath it

This is my HTML:

<h3>test 1</h3>
<p>blah</p>
<h4>subheading 1</h4>
<p>blah</p>
<h4>subheading 2</h4>
<h3>test 2</h3>
<h4>subheading 3</h4>
<p>blah</p>
<h3>test 3</h3>

I am trying to build an array of the h3 tags, with the h4 tags nested within them. An example of the array would look like:

Array
(
    [test1] => Array
        (
            [0] => subheading 1
            [1] => subheading 2
        )

    [test 2] => Array
        (
            [0] => subheading 3
        )

    [test 3] => Array
    开发者_StackOverflow中文版    (
        )

)

Happy to use preg_match or DOMDocument, any ideas?

---

With DOMDocument:

• use XPath "//h3" to find all <h3>. These will be the first-level entries in your array
• for each of them:
  • count a variable $i (count from 1!) as part of the loop
  • use XPath "./following::h4[count(preceding::h3) = $i]" to find any sub-ordinate <h4>
  • these will be second-level in you array

The XPath expression is "select all <h4> that have a the same constant number of preceding <h3>". For the first <h3> that count is 1, naturally, for the second the count is 2, and so on.

Be sure to execute the XPath expression in the context of the respective <h3> nodes.