open_basedir not working with PHP on SUPHP mode.
Workarround:
Copied open_basedir from textbox and put on custom php.ini as directive.
Sugest put a little checkbox near or before textarea on the same screen labeled "enable open_basedir to suphp". And this will be copy the line and create a custom php.ini automatically with openbase_dir current directories of website on basedir input field.