How to log pretty printed json in PHP?

Question

I'm building an API in php. This API processes json messages from a third party API.

I want to log invalid pretty printed json messages.

So I did this:

error_log("test\n" . json_encode($json_string, JSON_PRETTY_PRINT));

However, when I look at my logs, the json string is not pretty printed:

$ tailf error.log
2015-07-13 10:20:03: (mod_fastcgi.c.2701) FastCGI-stderr: test
"{\"info\":{\"status\":200,\"msg\":\"OK\"},\"response\":{\"foo\":\"bar\"}"

I want to see something like:

$ tailf error.log
2015-07-13 10:20:03: (mod_fastcgi.c.2701) FastCGI-stderr: test
{
  "info": {
     "status": 200,
     "msg ": "OK"
  },
  "response": {
     "foo": "bar"
  }
}

How can I achive this result?

Answer 1

error_log("test\n" . json_encode($json_string, JSON_PRETTY_PRINT));

json_encode() will actually not necessarily produce JSON: it will produce something that can be read by javascript. If you give it an array or an object, it will produce JSON; if you give it a string, it will produce a javascript string. And that’s what you’re doing, so that’s what you’re getting.

To be clear, $json_string is a string: (as far as PHP is concerned, it’s a string; if you passed that same string to javascript, it would be interpreted as an object). You pass that through json_encode() and all you’re going to get is another string (a string of doubly-encoded JSON).

JSON_PRETTY_PRINT is having no effect here, because you’re not producing JSON: you’re producing something that javascript too would see as a string.

Savvy?

So what you need to do is to (a) turn $json_string back into a PHP array, and then (b) reencode that as JSON, this time using the JSON_PRETTY_PRINT flag.

$log_array = json_decode($json_string, true);
$json_pretty_string = json_encode($log_array, JSON_PRETTY_PRINT);
error_log('test' . PHP_EOL . $json_pretty_string);

Rather than converting it back to a PHP array and then back to JSON, it would be better to add the JSON_PRETTY_PRINT flag to wherever you’re getting $json_string from in the first place, if possible.

---

Alternatively, just log $json_string directly (no need to encode it: it’s already a string, you can pass it to error_log() as it is), and worry about prettifying it only when you need to read your logs. This will make your logs considerably smaller.

Answer 2

Common unix error logs are not supposed to contain human-readable json or other unescaped characters. Many syslog/logging implementations are limited by character width and automatically add encoding (like \") or remove new line characters, PHP's error_log is not binary safe either - the behaviour when encountering a unicode character is unpredictable tho (not sure).

You should not be using the native syslog/error log functions, instead, build your own logger, dedicated to json logging.

Personally I use MongoDB for logging json, because it's the kind of data MongoDB is supposed to work with.