xquery_to_add_strong_class.php

<?php
/**
 * Creates a DOM Document based on the content.
 *
 * @param string $content The post content.
 * @param string The modified HTML.
 */
function dom_add_strong_class( $content ) {

	// Create a new DOMDocument object.
	$doc = new DOMDocument();
	libxml_use_internal_errors( true );

	// Load the HTML content.
	$doc->loadHTML( $content, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD );

	$xpath = new DOMXPath( $doc );

	// Find all <strong> and <b> tag.
	$strong_tags = $xpath->query( '//b | //strong' );

	foreach ( $strong_tags as $strong_tag ) {
		if ( $strong_tag->hasAttribute( 'class' ) ) {
			$current_classes = $strong_tag->getAttribute( 'class' );
			$strong_tag->setAttribute( 'class', $current_classes . ' extra-strong' );
		} else {
			$strong_tag->setAttribute( 'class', 'strong' );
		}
	}
	// Save the modified HTML.
	$modified_html = $doc->saveHTML();

	return $modified_html;
}