Some old PHP code I use to minify HTML code.

minify-html.php

<?php

function ob_end_flush_minified() {
	$content = ob_get_contents();
	ob_end_clean();

	$content = minify_html($content);

	echo $content;
}

function minify_html_sub ($content) {
	$content = preg_replace('/[\s]+/'," ", $content);
	$content = preg_replace('~ (<(ul|/ul|ol|/ol|li|/li|div|/div|table|/table|td|/td|tr|/tr|th|/th|br|body|/body|html|/html|head|/head|h[123456]|/h[123456]|dl|/dl|dd|/dd|dt|/dt|form|/form|iframe|/iframe|hr|map|/map|area|/area|p|/p|pre|select|/select|option|/option|style)[^>]*>)~',"$1", $content);
	$content = preg_replace('~(<(ul|/ul|ol|/ol|li|/li|div|/div|table|/table|td|/td|tr|/tr|th|/th|br|applet|body|/body|html|/html|head|/head|h[123456]|/h[123456]|dl|/dl|dd|/dd|dt|/dt|form|/form|iframe|/iframe|hr|map|/map|area|/area|p|/p|/pre|selecy|/select|option|/option|/style)[^>]*>) ~',"$1", $content);
	$content = preg_replace('~(<script [^>]*><!--) *~',"$1\n", $content);
	$content = preg_replace('~ *(//--></script>)~',"\n$1", $content);

	return $content;
}

function minify_html ($buffer){
    $poz_current = 0;
    $poz_end = strlen($buffer)-1;
    $result = "";

    while ($poz_current < $poz_end){
        $end_tag = null;

		$t_poz_textarea_start = stripos($buffer, "<textarea", $poz_current);
		$t_poz_pre_start = stripos($buffer, "<pre", $poz_current);
		$t_poz_start = false;

		if (($t_poz_textarea_start !== false && $t_poz_pre_start === false) || ($t_poz_textarea_start !== false && $t_poz_pre_start !== false && $t_poz_textarea_start < $t_poz_pre_start)) {
			$t_poz_start = $t_poz_textarea_start;
            $t_poz_end = stripos($buffer, "</textarea>", $t_poz_start);
			if ($t_poz_end === false) $t_poz_start = false;
		}

		if (($t_poz_pre_start !== false && $t_poz_textarea_start === false) || ($t_poz_pre_start !== false && $t_poz_textarea_start !== false && $t_poz_pre_start < $t_poz_textarea_start)) {
			$t_poz_start = $t_poz_pre_start;
            $t_poz_end = stripos($buffer, "</pre>", $t_poz_start);
			if ($t_poz_end === false) $t_poz_start = false;
		}

        if ($t_poz_start !== false) {
            $buffer_part_2strip = substr($buffer, $poz_current, $t_poz_start-$poz_current);
            $temp = minify_html_sub($buffer_part_2strip);
            $result .= $temp;
            $temp = substr($buffer, $t_poz_start, $t_poz_end-$t_poz_start);
            $result .= $temp;
            $poz_current = $t_poz_end;
        } else {
            $buffer_part_2strip = substr($buffer, $poz_current);
            $temp = minify_html_sub($buffer_part_2strip);
            $result .= $temp;
            $poz_current = $poz_end;
        }
    }
    return $result;
}