As the manuals states, the spl extension should be enabled by default.
The new dev-lang/php ebuilds work on the assumption that everything can be
disabled and that's the default state: all disabled. Then you enable yourself
any extension you need, in fact SPL is not _needed_ for PHP5 to work, but it
greatly helps, so in the end we leave the user the choice of enabling it or not.
Best regards, CHTEKK.