Description:
------------
The problem seems to occur on both Windows and Linux, using any version from 4.3.2 or higher (5.x.x not included). When I define an output_handler in my php.ini of .htaccess and turn output-buffering on, the output_handler is not called before sending the output to the browser.

Reproduce code:
---------------
[.htaccess]:
php_value auto_prepend_file foo.php
php_flag output_buffering on
php_value foo

[prepend.php]
foo(&$buffer)
{
    return "foo" . $buffer;
}

[bar.php]
echo "bar";

Expected result:
----------------
foobar

Actual result:
--------------
bar