<?php
/**
 * @param string
 * @param string
 * @param string
 * @return string
 */
function removeElementsByClass($html, $class, $tag_name='*')
{
	if (stripos($html, '<html') === false)
		return $html;

	foreach (['tag_name', 'class'] as $arg) {
		if (!is_string($$arg) || empty($$arg))
			throw new InvalidArgumentException("Expected non-empty string \$$arg");
	}

	$doc = new DOMDocument('1.0', 'utf-8');
	$doc->loadHTML($html);

	$xpath = new DOMXPath($doc);
	$query = sprintf('//%s[contains(attribute::class, "%s")]', $tag_name, $class);
	foreach($xpath->query($query) as $e)
		$e->parentNode->removeChild($e);

	return $doc->saveHTML($doc->documentElement);
}

/**
 * WordPress example: remove all elements with class `remove-for-mobile`
 * if user agent is considered mobile.
 */
if (!is_admin() && wp_is_mobile()) {
	add_action('after_setup_theme', function()
	{
		ob_start(function($output)
		{
			return removeElementsByClass($output, 'remove-for-mobile');
		});
	}, 0);
	add_action('shutdown', 'ob_end_flush', PHP_INT_MAX);
}