Description:
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json_encode(array("\x80" => 1)) returns "{null: 1}" in PHP5.3 and PHP5.4. It's not a valid JSON, but at least we could use json_last_error() to catch it. Encoding recursion, INF/NAN or unsupported type results in a VALID json so that json_last_error() returns 0 (no error) and only emit E_WARNING. 
In fact, all JSON_ERROR_* constants are only for json_decode() until 5.3.3.

From PHP 5.5, json_encode() does not emit E_WARNING anymore,  JSON_ERROR_RECURSION/INF_OR_NAN/UNSUPPORTED_TYPE are added and json_last_error() may return them after json_encode(). (And it also possible return JSON_ERROR_DEPTH for the new third param of json_encode.)

The problem is, there is NO way to be tolerant to these cases in PHP 5.5+!
The undocumented option JSON_PARTIAL_OUTPUT_ON_ERROR is USELESS because json_last_error() only returns the last error, means there may be utf-8 error which results in invalid json.


There are two possible solution:
1. When json_encode with JSON_PARTIAL_OUTPUT_ON_ERROR, encode array("\x80" => 1) to {"": 1} or {} (just drop the node) to ensure valid json output.
2. Implement new options like https://bugs.php.net/bug.php?id=65082 , and add more options to allow indicate how to treat recursion, Inf, NaN, unsupported type respectively.

We'd better implement both, 1 for backward compatibility and 2 for drop unreliable json_last_error().