PHP中如何替换html中的内容呢

Question

<?php
header("Content-type:text/html;charset=utf-8");class template {
    public $_var;    
    public $_content;
    public $_fileName = 'index.html';
    public $left_delimeter = '[[';
    public $right_delimeter = ']]';
    
    function getContent($file_name) {
        return $this->_content = file_get_contents($file_name);
    }
    function assign($key, $value) {
        if (!empty($key))
            $this->_var[$key] = $value;
    }
    function display() {
        $this->getContent($this->_fileName);        
        preg_match_all('/\[\[(.*?)\]\]/', $this->_content, $match);
        if (!empty($match[1]) && !empty($match[0])) {
            foreach($match[1] as $tag) {                
                $this->_content = str_replace($this->left_delimeter . $tag . $this->right_delimeter, $this->_var[$tag], $this->_content);                                        
            }
        }
        echo $this->_content;
    }
}$t = new template();
$t->assign('LEFT_BLOCK', '<div id="left_block" style="width:200px;height:200px;background-color:#00f;">Left</div>');
$t->assign('RIGHT_BLOCK', '<div id="right_block" style="margin-top:20px;width:200px;height:200px;background-color:#00f;">Right</div>');
$t->display();