Php 确定相对于文档根的文件路径

我有一个符号链接

/var/www/sitename/html/lib/

，它指向

/var/www/sitename/application/lib/

，而

文档的根是
/var/www/sitename/html/

在
/var/www/sitename/application/lib/testing/relative.php
中，我希望找到该文件相对于文档根的路径（即
/lib/testing/relative.php
）。请注意，
relative.php
包含一个类，不直接调用

如果从同样位于符号链接中的文件调用
relative.php
，例如，我可以找到相对于文档根的路径，但是，我无法从

在不硬编码符号链接路径的情况下，如果从非符号链接中的文件调用，是否可以找到相对于
文档根的
/var/www/sitename/application/lib/testing/relative.php的路径

下面是我尝试过的

/var/www/sitename/lib/testing/relative.php

<?php
class relative {
    public function __construct()
    {
        $__FILE__=__FILE__;
        $PHP_SELF=$_SERVER['PHP_SELF'];
        $DOCUMENT_ROOT=$_SERVER['DOCUMENT_ROOT'];
        $SCRIPT_FILENAME=$_SERVER['SCRIPT_FILENAME'];
        $SCRIPT_FILENAME_REAL=realpath($_SERVER['SCRIPT_FILENAME']);

        $replace1=str_replace($PHP_SELF,'',$SCRIPT_FILENAME_REAL);
        $replace2=str_replace($replace1,'',__FILE__);

        echo("__FILE__              => $__FILE__\n");
        echo("PHP_SELF              => $PHP_SELF\n");
        echo("DOCUMENT_ROOT         => $DOCUMENT_ROOT\n");
        echo("SCRIPT_FILENAME       => $SCRIPT_FILENAME\n");
        echo("SCRIPT_FILENAME_REAL  => $SCRIPT_FILENAME_REAL\n\n");
        echo('str_replace($PHP_SELF,"",$SCRIPT_FILENAME_REAL)'."\n");
        echo('$replace1             => '.$replace1."\n\n");
        echo('str_replace(str_replace($PHP_SELF,"",$SCRIPT_FILENAME_REAL),"",__FILE__)'."\n");
        echo('$replace2             => '.$replace2);

    }
}
?>

/var/www/sitename/testing.php

<?php
require_once('testing/relative.php');
new relative();  
?>

<?php
require_once('lib/testing/relative.php');
new relative();  
?>

<?php
require_once('lib/testing/relative.php');
new relative();  
?>

__FILE__              => /var/www/sitename/application/lib/testing/relative.php
PHP_SELF              => /testing.php
DOCUMENT_ROOT         => /var/www/sitename/html
SCRIPT_FILENAME       => /var/www/sitename/html/testing.php
SCRIPT_FILENAME_REAL  => /var/www/sitename/html/testing.php

str_replace($PHP_SELF,"",$SCRIPT_FILENAME_REAL)
$replace1             => /var/www/sitename/html

str_replace(str_replace($PHP_SELF,"",$SCRIPT_FILENAME_REAL),"",__FILE__)
$replace2             => /var/www/sitename/application/lib/testing/relative.php