Description:
------------
A script that does file_get_contents or file in the callback function of ob_start wont work. The file_get_contents function cant open the file. 

<?php 
    // Ob callback 
    function ob($buffer) 
    { 
        return file_get_contents('open.txt'); 
    } 
    ob_start('ob'); 
?>

The behaviour is so because the callback sets its cwd to the root of the disk instead of the folder the script was called from. 

If I copy the file I want to open to my root it can open, otherwise it will throw me this error.

Warning: file_get_contents(open.txt): failed to open stream: No such file
or directory in /var/www/test.php on line 6 

Specifying the full path works. Could be a possible bug in the callback, otherwise a documentation problem because I was unable to find it anywhere in the docs.


Reproduce code:
---------------
<?php 
    // Ob callback 
    function ob($buffer) 
    { 
        return file_get_contents('open.txt'); 
    } 
    ob_start('ob'); 
?>

Expected result:
----------------
The content of open.txt

Actual result:
--------------
Warning: file_get_contents(open.txt): failed to open stream: No such file
or directory in /var/www/test.php on line 6