WordPress: Add custom TinyMCE button that wraps selection in given tag with a couple extra functions from within your theme’s functions.php

functions.php

<?php

/**
 * Filter TinyMCE wysiwig editor to include custom button
 *
 * @since MyTheme 1.0
 *
 * @param $buttons TinyMCE button array
 * @return $buttons TinyMCE button array
 */
function mytheme_mce_buttons_2( $buttons ) {
	/**
	 * Add in our custom wrapping button
	 */
	$buttons[] = 'wrap';

	return $buttons;
}
add_filter( 'mce_buttons_2', 'mytheme_mce_buttons_2' );

/**
 * Add custom button handler to TinyMCE settings JS
 *
 * @since MyTheme 1.0
 *
 * @param $settings array of TinyMCE settings
 * @return $settings array of TinyMCE settings
 */
function mytheme_tinymce_settings( $settings ) {
	// Add a setup function which registers our simple button to wrap selection in a <div> with class="customclass"
	$setup_function = 'function(ed) {
		ed.addButton("wrap", {
			title : "Wrap selected content in div.customclass",
			image : "' . get_template_directory_uri() . '/img/wrap-icon.png",
			onclick : function() {
				ed.focus();
				ed.selection.setContent("<div class=customclass>" + ed.selection.getContent() + "</div>");
			}
		});
	}';
	// Strip out new lines (they will break the JS) and add the setup function
	$settings['setup'] = str_replace( "\n", '', $setup_function );
	return $settings;
}
add_filter( 'tiny_mce_before_init', 'mytheme_tinymce_settings' );