php截取指定字符串之间的字符串的类

<?php  
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;  
    }  
}  
?>  

上述类的使用方法：
<?php
    $get_c_str = new get_c_str;
    echo $get_c_str -> get_str('a123456789b','a','b');
?>