Fix PHP_EXTRA_VERSION.

Summary: This is a continuation of #2943.
Closes #2970

Reviewed By: @LiraNuna

Differential Revision: D1393799

Pulled By: @JoelMarcey
set default thread count to cpu * 2

Summary: A heuristic invented by @joelpob. Thoughts?

Reviewed By: @jdelong

Differential Revision: D1392472
Changed errormessage in implode()

Summary: It errors if none of the arguments is a container.
Added the function name to it to be more clear

Resolves #2567
Closes #2974

Reviewed By: @ptarjan

Differential Revision: D1393809

Pulled By: @JoelMarcey