bigsweater/heading_anchors.php

## heading_anchors.php
<?php

// Obviously remove namespace stuff if you're not using namespaces.

/**
 * Add anchor IDs to headings in content
 */
function anchor_headings( $content ) {
  $charset = '';

  if ( defined('DB_CHARSET') ) {
    $charset = DB_CHARSET;
  } else {
    $charset = 'UTF-8';
  }

  $doc = new \DOMDocument();
  $doc -> LoadHTML( mb_convert_encoding($content, 'HTML-ENTITIES', $charset) );
  $doc -> strictErrorChecking = false;
  $doc -> formatOutput = true;
  $xpath = new \DOMXPath( $doc );

  $headings = $xpath -> query( '//h1 | //h2 | //h3 | //h4 | //h5 | //h6' );

  $i = 0;

  foreach ( $headings as $heading ) {
    $slug = sanitize_title_with_dashes( $heading -> textContent, null, 'save' ); // The 'save' param tells WP to strip out all special characters so you don't get HTML entities in your IDs.
    $slug .= '-' . $i;

    $i++;

    $heading -> setAttribute( 'id', $slug );
  }

  // Setting arguments on LoadHTML caused PHP to strip out some HTML tags. So
  // instead of this: http://stackoverflow.com/a/22490902/343520
  // I had to do this: http://stackoverflow.com/a/6953808/343520
  // It keeps my encoding and all my tags intact, even if they're malformed.
  // remove <!DOCTYPE
  $doc -> removeChild( $doc -> doctype);

  // remove <html><body></body></html>
  $doc -> replaceChild( $doc->firstChild->firstChild, $doc->firstChild );

  return $doc -> saveHTML();
}
add_filter( 'the_content', __NAMESPACE__ . '\\anchor_headings' );
	<?php

	// Obviously remove namespace stuff if you're not using namespaces.

	/**
	* Add anchor IDs to headings in content
	*/
	function anchor_headings( $content ) {
	$charset = '';

	if ( defined('DB_CHARSET') ) {
	$charset = DB_CHARSET;
	} else {
	$charset = 'UTF-8';
	}

	$doc = new \DOMDocument();
	$doc -> LoadHTML( mb_convert_encoding($content, 'HTML-ENTITIES', $charset) );
	$doc -> strictErrorChecking = false;
	$doc -> formatOutput = true;
	$xpath = new \DOMXPath( $doc );

	$headings = $xpath -> query( '//h1 \| //h2 \| //h3 \| //h4 \| //h5 \| //h6' );

	$i = 0;

	foreach ( $headings as $heading ) {
	$slug = sanitize_title_with_dashes( $heading -> textContent, null, 'save' ); // The 'save' param tells WP to strip out all special characters so you don't get HTML entities in your IDs.
	$slug .= '-' . $i;

	$i++;

	$heading -> setAttribute( 'id', $slug );
	}

	// Setting arguments on LoadHTML caused PHP to strip out some HTML tags. So
	// instead of this: http://stackoverflow.com/a/22490902/343520
	// I had to do this: http://stackoverflow.com/a/6953808/343520
	// It keeps my encoding and all my tags intact, even if they're malformed.
	// remove <!DOCTYPE
	$doc -> removeChild( $doc -> doctype);

	// remove <html><body></body></html>
	$doc -> replaceChild( $doc->firstChild->firstChild, $doc->firstChild );

	return $doc -> saveHTML();
	}
	add_filter( 'the_content', __NAMESPACE__ . '\\anchor_headings' );