Yesterday we released the new version of the MongoDB Driver for PHP, to coincide with the release of MongoDB 3.4. Not long after that, we received an issue through GitHub titled "Undefined Symbol php_json_serializable_ce in Unknown on Line 0".
The driver makes use of the JSON extension's "JsonSerializable" interface to handle some of the BSON types (like binary data). They were surprised that, despite running their tests on a wide range of builds they never came up with this same issue, compiling them from source. The key here is that the JSON extension is bundled along with the binary when compiled this way however some linux distributions do things differently. They ship it as a separate module and, because this could potentially be missing, a JSON error like the one reported could occur. He goes on to talk about some specific examples from various distributions and the simple fix - ensure the JSON extension is loaded before the MongoDB driver is loaded in your installation. This prevents the JSON handling from being missing and the JSON-related error message from popping up.