php字符串随意截取问题

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;
}
}

$get_c_str = new get_c_str;
echo $get_c_str -> get_str('北京市丰台区','市','区');