How to minify HTML using WordPress functions.php

functions--wordpress-minify-class.php

<?php
/*********
	*********
	Minify
	*********
*********/

class WP_Minify{
	protected $compress_css = true;
	protected $compress_js = false;
	protected $img_to_base64 = true;
	protected $remove_comments = false;
	protected $html;
	public function __construct($html){
		if(!empty($html)){
			$this->parseHTML($html);
		}
	}
	public function __toString(){
		return $this->html;
	}
	protected function minifyHTML($html){
		$pattern = '/<(?<script>script).*?<\/script\s*>|<(?<style>style).*?<\/style\s*>|<!(?<comment>--).*?-->|<(?<tag>[\/\w.:-]*)(?:".*?"|\'.*?\'|[^\'">]+)*>|(?<text>((<[^!\/\w.:-])?[^<]*)+)|/si';
		preg_match_all($pattern, $html, $matches, PREG_SET_ORDER);
		$overriding = false;
		$raw_tag = false;
		$html = '';
		foreach($matches as $token){
			$tag = isset($token['tag']) ? strtolower($token['tag']) : null;
			$content = $token[0];
			if(is_null($tag)){
				if(!empty($token['script'])){
					$strip = $this->compress_js;
				} else if(!empty($token['style'])){
					$strip = $this->compress_css;
				} else if($this->remove_comments){
					if(!$overriding && $raw_tag != 'textarea'){
						$content = preg_replace('/<!--(?!\s*(?:\[if [^\]]+]|<!|>))(?:(?!-->).)*-->/s', '', $content);
					}
				}
			} else{
				if($tag == 'pre' || $tag == 'textarea' || $tag == 'script'){
					$raw_tag = $tag;
				} else if($tag == '/pre' || $tag == '/textarea' || $tag == '/script'){
					$raw_tag = false;
				} else if($tag == 'img' && $this->img_to_base64){
					$content = preg_replace_callback('/(<img\s+src=["\'])([^"\']+)(["\']\s+[^>]+>)/', function($matches){
						$file = file_get_contents($matches[2]);
						$file_info = new finfo(FILEINFO_MIME_TYPE);
						$mime = $file_info->buffer($file);
						$data = base64_encode($file);
						return $matches[1] . "data:$mime;base64,$data" . $matches[3];
					}, $content);
				} else{
					if($raw_tag || $overriding){
						$strip = false;
					} else{
						$strip = true;
						$content = preg_replace('/(\s+)(\w++(?<!\baction|\balt|\bcontent|\bsrc)="")/', '$1', $content);
						$content = str_replace(['/>', ' />', ' >'], '>', $content);
					}
				}
			}
			if($strip){
				$content = $this->removeWhiteSpace($content);
			}
			$html .= $content;
		}
		return $html;
	}
	public function parseHTML($html){
		$this->html = $this->minifyHTML($html);
	}
	protected function removeWhiteSpace($str){
		$str = str_replace(['  ', '&nbsp;'], ' ', $str);
		$str = str_replace(["\t", "\n", "\r"], '', $str);
		return $str;
	}

}
add_action('get_header', function(){
	ob_start(function($html){
		return new WP_Minify($html);
	});
});