Despite the misleading title, this post on Reddit talks some about a switch that some Linux distributions are making when it comes to JSON support in PHP. They're moving away from the built-in support in favor of including this one.
In a quote from Nikita Popov (a comment on the post) he notes that:
It is true that some Linux distribution switched from json to json-c, but this should be transparent to the user. The standard PHP distribution still ships the JSON extension as it always did. [...] You should all take this chance to switch to PHP 5.5, so you can see that everything works fine and that PHP 5.5 is awesome
He also includes comments from the Remi (Fedora) project about the switch, noting that the end user shouldn't notice any kind of issues. The reasoning behind the switch has to do with licensing and usage issues of the previously built-in extension. You can find out more about that issue in this bug report.