class get_c_str {

var $str;

var $start_str;

var $end_str;

var $start_pos;

var $end_pos;

var $c_str_l;

var $contents;

function get_str($str,$start_str,$end_str){

   $this->str = $str;

   $this->start_str = $start_str;

   $this->end_str = $end_str;

   $this->start_pos = strpos($this->str,$this->start_str)+strlen($this->start_str);

     $this->end_pos = strpos($this->str,$this->end_str);

   $this->c_str_l = $this->end_pos - $this->start_pos;

   $this->contents = substr($this->str,$this->start_pos,$this->c_str_l);

   return $this->contents;

}

}

//使用方法

$url = 'http://www.xiaodoubi.com';

$data = file_get_contents($url);

$get_c_str = new get_c_str;

$content = $get_c_str -> get_str($data,'<p class="message">','</p>');

echo $content;