为什么我的getCaptcha()写完如果输出图像就没办法返回$str字符串呢？

 public function getCaptcha(){
        //设置填充颜色
      $white=imagecolorallocate($this->_image,255,255,255);
        //绘制填充矩形
      imagefilledrectangle($this->_image, 0, 0, $this->_width,$this->_height,$white);
        //获取字符
      $str=$this->_getStr();
      if($str === false){
          return false;
      }
        //绘制验证码
      $fontfile=$this->_fontfile;
      for($i=0;$i<$this->_length;$i++){
          $size=$this->_size;
          $fonheight=imagefontheight($size);
          $angle=mt_rand(-15,15);
          $x=20+ceil($this->_width/$this->_length)*$i;
          $y=mt_rand($this->_height/2,$this->_height-$fonheight);
          $color=$this->_getRoundColor();
          //$text=mb_substr($str,$i,1,'utf-8');//从字符集从随机获取一个字符
          $text=$str{$i};
          imagettftext($this->_image, $size, $angle, $x, $y, $color, $fontfile, $text);
      }
      //绘制干扰元素 分为雪花模式和像素点线段模式
      if($this->_snow){
          $this->_getSnow();
      }else{
          if($this->_pixel){
              $this->_getPixel();
          }
          if($this->_line){
              $this->_getLine();
          }
      }
      //输出图像
      /*向浏览器输出图像*/
      header('content-type:image/png');
      imagepng($this->_image);
      imagedestory($this->_image);
      return strtolower($str);
  }