package/php: security bump version to 5.6.18

Changelog: http://www.php.net/ChangeLog-5.php#5.6.18


Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>