How I added GeSHi to GPORG. Here I will demonstrate using GeSHi (for prettification purposes).

<?php
 
function sexy_code ($a, $content) {
	$attr = shortcode_atts(array(				// sets the defaults for each attribute so we don't run into errors if we're missing any
		'lang' => 'html',						// default language is HTML
		'start' => '1'							// which line num to start on
	), $a);
 
	$content = strip_tags($content);			// remove wpautop() formatting from the snippet
 
	$array = explode("\n", $content);
	$tab = null;
	$t = 0;
	$pos = null;
 
	foreach ($array as $i => $line) {
		$code = $line;
		if (strpos($line, '//') !== false) {
			list($code, $comment) = explode('//', $line);
		}
			if (preg_match("#\{|\($#", $code)) {
			$pos = 'after';
			$t++;
		}
		elseif (preg_match("#\}|\)$#", $code)) {
			$pos = 'before';
			$t--;
		}
		$tab = ($t > 0 ? implode("", array_fill(0, $t, "    ")) : null);
		$line = ($pos == 'before' ? $tab : null).$line.( $pos == 'after' ? $tab : null);
		$array[$i] = $line;
	}
	$content = implode("\n", $array);
	$content = preg_replace("#(    +)(\n)#", "\\2\\1", $content);
	$content = str_replace("    }", "}", $content);
 
	$content = preg_replace("#\&amp;([a-z0-9]+);#", "&\\1;", $content);
	$content = str_replace('&lt;', '<', $content);
	$content = str_replace('&gt;', '>', $content);
 
	//echo secret_output ('test HTML',$content);
 
	$lang = $attr['lang'];
	$geshi = new GeSHi($content, $lang);
	$geshi->enable_line_numbers(GESHI_FANCY_LINE_NUMBERS);	// type of line numbers
	$geshi->enable_classes();					// turns on classes so we can prettify thru CSS
		if ($attr['start'] != 1)				// if you specifically mentioned which line number you want the snippet to start on
	$geshi->start_line_numbers_at($attr['start']);		// ...set it here
 
	$r = $geshi->parse_code();
 
	$r = str_replace('<ol>', '<ol class="no-custom">', $r);
	$r = preg_replace("#^<pre>#", '<pre class="wp-block-code">', $r);
 
	return $r;
}
add_shortcode( 'code', 'sexy_code' );			// shortcodify!
 
?>

(See the doc on WordPress’s shortcode API if you haven’t already.)

To use in a post…

[code lang="php" start="2"]<?php echo $fun; ?>[/code]

…will get you…

<?php echo $fun; ?>