How to strip a tag and all of its inner html using the tag's id?

With native DOM

$dom = new DOMDocument;
$dom->loadHTML($htmlString);
$xPath = new DOMXPath($dom);
$nodes = $xPath->query('//*[@id="anotherDiv"]');
if($nodes->item(0)) {
    $nodes->item(0)->parentNode->removeChild($nodes->item(0));
}
echo $dom->saveHTML();

You can use preg_replace() like:

$string = preg_replace('/<div id="someid"[^>]+\>/i', "", $string);

Using the native XML Manipulation Library

Assuming that your html content is stored in the variable $html:

$html='<html>
 <body>
 bla bla bla bla
  <div id="myDiv"> 
         more text
      <div id="anotherDiv">
           And even more text
      </div>
  </div>

  bla bla bla
 </body>
</html>';

To delete the tag by ID use the following code:

    $dom=new DOMDocument;

    $dom->validateOnParse = false;

    $dom->loadHTML( $html );

    // get the tag

    $div = $dom->getElementById('anotherDiv');

   // delete the tag

    if( $div && $div->nodeType==XML_ELEMENT_NODE ){

        $div->parentNode->removeChild( $div );
    }

    echo $dom->saveHTML();

Note that certain versions of libxml require a doctype to be present in order to use the getElementById method.

In that case you can prepend $html with <!doctype>

$html = '<!doctype>' . $html;

Alternatively, as suggested by Gordon's answer, you can use DOMXPath to find the element using the xpath:

$dom=new DOMDocument;

$dom->validateOnParse = false;

$dom->loadHTML( $html );

$xp=new DOMXPath( $dom );

$col = $xp->query( '//div[ @id="anotherDiv" ]' );

if( !empty( $col ) ){

    foreach( $col as $node ){

        $node->parentNode->removeChild( $node );

    }

}

echo $dom->saveHTML();

The first method works regardless the tag. If you want to use the second method with the same id but a different tag, let say form, simply replace //div in //div[ @id="anotherDiv" ] by '//form'