如何测试程序执行到哪里

请问一下各位高手，如何测试程序执行到哪里呢？你们平时在研究人家源码的时候在这方面都有哪些技巧呢？我现在在学习ecshop的二次开发，这个东西要看懂确实不容易啊，需要很多调试技巧，不知道各位高手都有和高招，比如    function upload_image($upload, $dir = '', $img_name = '')
    {
        /* 没有指定目录默认为根目录images */
        if (empty($dir))
        {
            /* 创建当月目录 */
            $dir = date('Ym');
            $dir = ROOT_PATH . $this->images_dir . '/' . $dir . '/';
            
        }
        else
        {
            /* 创建目录 */
            $dir = ROOT_PATH . $this->data_dir . '/' . $dir . '/';
            
            if ($img_name)
            {
                $img_name = $dir . $img_name; // 将图片定位到正确地址
              
            }
        }

        /* 如果目标目录不存在，则创建它 */
        if (!file_exists($dir))
        {
    include("lib_base.php");
            if (!make_dir($dir))
            {
                /* 创建目录失败 */
                $this->error_msg = sprintf($GLOBALS['_LANG']['directory_readonly'], $dir);
                $this->error_no  = ERR_DIRECTORY_READONLY;

                return false;
            }
        }

        if (empty($img_name))
        {
            $img_name = $this->unique_name($dir);
            $img_name = $dir . $img_name . $this->get_filetype($upload['name']);
        
        }

        if (!$this->check_img_type($upload['type']))
        {
            $this->error_msg = $GLOBALS['_LANG']['invalid_upload_image_type'];
            $this->error_no  =  ERR_INVALID_IMAGE_TYPE;
            return false;
        }

        /* 允许上传的文件类型 */
        $allow_file_types = '|GIF|JPG|JEPG|PNG|BMP|SWF|';
        if (!check_file_type($upload['tmp_name'], $img_name, $allow_file_types))
        {
            $this->error_msg = $GLOBALS['_LANG']['invalid_upload_image_type'];
            $this->error_no  =  ERR_INVALID_IMAGE_TYPE;
            return false;
        }

        if ($this->move_file($upload, $img_name))
        {
            return str_replace(ROOT_PATH, '', $img_name);
        }
        else
        {
            $this->error_msg = sprintf($GLOBALS['_LANG']['upload_failure'], $upload['name']);
            $this->error_no  = ERR_UPLOAD_FAILURE;

            return false;
        }
    }


上面这个函数里面有return false 我调试的时候，它只执行了一半，但是又不知道它执行到哪里。。。。该怎么测试呢？