To set a standart auto_prepend_file for php on a dedicated root server is not a bit thing but on a virtual webhosting or managed server it could be a big issue. Because you dont have systemwide global access to httpd.conf and php.ini files.
My small solutions for Hostrurope managed virtual hosting packets:
Add this line to your .htaccess file
And be sure that your auto_prepend.php really exists and its path is correct.
To open a text file and edit it on Debian use "vim" command.
vim mytextfile.txt
To save changes and exit :
Press "Esc" key on your keyboard.
And give "wq" and press Enter.
To quit without save changes ESC+q! than Enter