tag.php

<?php
class Tag {
	private $name;
	private $attributes = [];
	private $content = [];
	
	public function __construct($name) {
		$this->name = $name;
	}
	
	public function attr($key, $value) {
		$this->attributes[$key] = $value;
		return $this;
	}
	
	public function append($content) {
		$this->content[] = $content;
		return $this;
	}
	
	public function __toString() {
		$attributes = '';
		foreach ($this->attributes as $key => $value) {
			$attributes .= ' ' . $key . '="' . htmlspecialchars($value, ENT_QUOTES) . '"';
		}
		$result = "<{$this->name}{$attributes}>";
		foreach ($this->content as $content) {
			$result .= $content;
		}
		$result .= "</{$this->name}>";
		return $result;
	}
}

class Ul extends Tag {
	public function __construct() {
		parent::__construct('ul');
	}
}

class Li extends Tag {
	public function __construct() {
		parent::__construct('li');
	}
}

$ul = new Ul();
$ul->append((new Li())->append('List item 1')->attr('class', 'list-item'));
$ul->append((new Li())->append('List item 2')->attr('class', 'list-item'));
$ul->append((new Li())->append('List item 3')->attr('class', 'list-item'));
echo $ul;