Simple WordPress Footnotes

footnotes.php

<?php
function api_footnotes( $content ) {
	$post_id = get_the_ID();
	
	//Need to correct wpautop() which smart-quoteify's the " in the numoffset argument. 
	$content = preg_replace('/numoffset=&#8221;(\d+)&#8243;/i', 'numoffset="$1"', $content);
	
	//Microsoft has some weird space characters that Mac/Unix systems don't have. What the next line does is replace the weird space characters with a real space character which makes the regex work...
	$content = str_replace('[ref ', '[ref ', $content);
	
	$start = 1;
	$notes = array();
	if (preg_match_all('/\[(ref((\s+)?numoffset="(\d+)+")? (.*?))\]/s', $content, $matches) ) {
		
		/*
		Given [0. numoffset="5" This is a footnote]
		
		$matches[0] = The whole match including the square brackets: [0. numoffset="5" This is a footnote]
		$matches[4] = numoffset value: 5
		$matches[5] = The footnote text: This is a footnote
		*/
		
		foreach( $matches[0] as $index => $target ) {
			$offset_value = (int) $matches[4][$index];
			$text = trim( $matches[5][$index] );
			
			//Footnotes that have [ or ] in the text break. Use double curly quotes as an escape to workaround this.
			$text = str_replace('{{', '[', $text);
			$text = str_replace('}}', ']', $text);
			
			
			if( $offset_value > 0 ) {
				$start = $offset_value;
			}
			
			$notes[] = $text;
		}
		
		$n = $start;
		foreach( $matches[0] as $index => $target ) {
			$content = str_replace($target, "<sup class=\"footnote\"><a href=\"#footnote-$post_id-$n\" id=\"fnref-$post_id-$n\">$n</a></sup>", $content);
			$n++;
		}

		// *****************************************************************************************************
		// Workaround for wpautop() bug. Otherwise it sometimes inserts an opening <p> but not the closing </p>.
		// There are a bunch of open wpautop tickets. See 4298 and 7988 in particular.
		$content .= "\n\n";
		// *****************************************************************************************************

		$content .= "<div class='footnotes'>";
		$content .= '<ol start="' . $start . '">';
		$i = $start;
		foreach($notes as $note) {
			$content .= "<li id=\"footnote-$post_id-$i\">$note <span class=\"footnotereverse\"><a href=\"#fnref-$post_id-$i\">&#8617;</a></span></li>";
			$i++;
		}
		$content .= "</ol>";
		$content .= "</div>";
		
		wp_enqueue_script( 'api-footnotes' );
	}

	return $content ;
}

add_filter('the_content', 'api_footnotes', 98); //Set priority to 98 so links are converted to footnotes before Yoast's analytics injects outbound tracking events.