content-heading-auto-id.php

<?php

/**
 * Content heading auto ID
 * 
 * Examples;
 * <h*>Test Header</h*> 
 * <h* id="test-header">Test Header</h*>
 */

add_filter( 'the_content', 'content_heading_filter' );

function content_heading_filter( $content )  {
	return $content = preg_replace_callback("#<(h[1-6])>(.*?)</\\1>#", "re_title", $content);
}

function re_title($match) {
	list($_unused, $h2, $title) = $match;
	$id = sanitize_title($title);
	return "<$h2 id='$id'>$title</$h2>";
}