lessons learned

Some HTML DOM parsing gotchas in PHP’s DOMDocument

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Although I had used Simple HTML DOM parser for WP DoNotTrack, I’ve been looking into native PHP HTML DOM parsing as a possible replacement for regular expressions for Autoptimize as proposed by Arturo. I won’t go into the performance comparison results just yet, but here’s some of the things I learned while experimenting with DOMDocument which in turn might help innocent passers-by of this blogpost.

  • loadHTML doesn’t always play nice with different character encodings, you might need something like mb_convert_encoding to work around that.
  • loadHTML will try to “repair” your HTML to make sure an XML-parser can work with it. So what goes in will not come out the same way.
  • loadHTML will spit out tons of warnings or notices about the HTML not being XML; you might want to suppress error-reporting by prepending the command with an @ (e.g. @$dom->loadHTML($htmlstring);)
  • If you use e.g. getELementsByTagName to extract nodes into a seperate DomNodeList and you want to use that to change the DomDocument can result in … unexpected behavior as the DomNodeList gets updated when changes are made to the DomDocument. Copy the DomNodes from the DomNodeList into a new array (which will not get altered) and iterate over that to update the DomDocument as seen in the example below.
  • removeChild is a method of DomNode, not of DomDocument. This means $dom->removeChild(DomNode) will not work. Instead invoke removeChild on the parent of the node you want to remove as seen in the example below
// loadHTML from string, suppressing errors
$dom = new DOMDocument();
@$dom->loadHTML($html);
// get all script-nodes
$_scripts=$dom->getElementsByTagName("script");
// move the result form a DomNodeList to an array
$scripts = array();
foreach ($_scripts as $script) {
   $scripts[]=$script;
}
// iterate over array and remove script-tags from DOM
foreach ($scripts as $script) {
   $script->parentNode->removeChild($script);
}
// write DOM back to the HTML-string
$html = $dom->saveHTML();

Now chop chop, back to my code to finish that performance comparison. Who know what else we’ll learn 😉