Fix fatal error because var_export doesn't handle circular dependency.

Fix fatal error because var_export doesn't handle circular dependency.

In logs there were lines such as:
PHP Fatal error:  Nesting level too deep - recursive dependency? in /home/web/prod/platal/include/security.inc.php on line 108, referer: https://www.polytechnique.org/register

More information can be found at:
- http://stackoverflow.com/questions/12488661/is-it-possible-to-list-out-all-the-global-variables-in-php
- http://php.net/manual/fr/function.var-export.php (in the comments)

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>