On line 59 (https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Process/ExecutableFinder.php#L59) ExecutableFinder checks if paths in open_basedir php config are directories.
This check throws a php warning if one path is non-existent.

The only solution I can think about it is to suppress warning with the @ operator.

For reference: https://bugs.php.net/bug.php?id=69240