thinkphp (3.2.1), write the source code a little bit about the problem. About absolute addresses.

thinkphp (3.2.1), write the source code a little bit of a problem. About the absolute address.
In accordance with the official source code, I knocked over the control. Just started knocking. I found a little problem.
First I wrote that thinkphp.php is the public entrance file.

 // +----------------------------------------------------------------------
// | thinkphp [WE CAN do it JUST THINK it]
// +----------------------------------------------------------------------
// | Copyright (c) 2006-2013 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed (http://www.apache.org/licenses/LICENSE-2.0)
// +----------------------------------------------------------------------
// | author:liu21st <[email protected]>
// +----------------------------------------------------------------------

//----------------------------------
thinkphp Public Entrance file
//----------------------------------

Record Start run time
$GLOBALS [' _begintime '] = Microtime (TRUE);
Recording Initial memory usage
Define (' memory_limit_on ', function_exists (' memory_get_usage '));
if (memory_limit_on) $GLOBALS [' _startusemems '] = Memory_get_usage ();

Version information
Const Think_version = ' 3.2.1 ';

URL Pattern Definition
Const Url_common = 0; Normal mode
Const URL_PATHINFO = 1; PathInfo mode
Const URL_REWRITE = 2; Rewrite mode
Const URL_COMPAT = 3; Compatibility mode

class file suffix
Const EXT = '. class.php ';

System constant Definition
Defined (' Think_path ') or define (' Think_path ', __dir__. ') /');
Echo Think_path;
Defined (' App_path ') or define (' App_path ', dirname ($_server[' script_filename ']). ' /');
Defined (' App_status ') or define (' app_status ', '); Apply state to load the corresponding configuration file
Defined (' App_debug ') or define (' App_debug ', false); Whether to debug mode

if (function_exists (' Saeautoloader ')) {//auto-Identify SAE environment
Defined (' App_mode ') or define (' App_mode ', ' SAE ');
Defined (' Storage_type ') or define (' Storage_type ', ' Sae ');
}else{
Defined (' App_mode ') or define (' App_mode ', ' common '); App mode defaults to normal mode
Defined (' Storage_type ') or define (' Storage_type ', ' File '); The storage type defaults to file
}

Defined (' Runtime_path ') or define (' Runtime_path ', App_path. ' runtime/'); System run-time directory
Defined (' Lib_path ') or define (' Lib_path ', Realpath (think_path. ' Library '). ' /'); System Core Class Library Directory

That's the line. Defines a constant lib_path. There was a problem. I first output think_path (35th line definition), output "d:\wamp\www\bbbb\thinkphp/" but when the constants are appended to the library, and then the Realpath function parsing, it will not be parsed, only the most back of the "/"。
Then I found the framework of the thinkphp, the source code, and then the public entrance file echo a bit, no problem.
However, when I directly copy this public access file to a new file, I get the same error. May I ask why this is?
There may be some confusion, I hope you don't mind.
------Solution--------------------
Not sure enough, don't modify the core code of the framework
When using frames, do not change any files outside of the file that allows you to modify

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