Fix byte compile error again; resurfaced in after a merge.

Cannot eval `php-extra-constants' at compile time as it is configured
at runtime by customize. Therefore, the php-constants const must be
evaluated at runtime.

Fixes error of the form:

php-mode.el:983:52:Error: Symbol's value as variable is void: php-extra-constants

Signed-off-by: Eric James Michael Ritz <lobbyjones@gmail.com>