You silently changed output of var_dump, but only mention var_export.
This is pretty serious BC break, it will break tons of tests for no good reason. Expect tons of failed tests on CI, you can see already failed ones here: https://travis-ci.org/php/php-src/jobs/211407088

I said it will break CI in current state, right? :)

Is there a BC promise on those methods other than output: string?

Also, that's reason why I did not target 5.6.x line

As @Majkl578 already pointed out, this change is going to break the world. The Travis build for php-src already comes in with ~2k failures (!!!), not to mention that pretty much all 3rd party extensions use phpt tests and as such rely on specific var_dump output. They would be in an even worse situation than php-src, because they would have to accept both output formats depending on the PHP version used.

This is too much breakage for an otherwise rather cosmetic change. As such, I'm closing this PR.

Thanks for reasoning Nikita.
I still do hope that one day it would be standardized.