json_last_error

While this can obviously change between versions, the current error codes are as follows:
0 = JSON_ERROR_NONE
1 = JSON_ERROR_DEPTH
2 = JSON_ERROR_STATE_MISMATCH
3 = JSON_ERROR_CTRL_CHAR
4 = JSON_ERROR_SYNTAX
5 = JSON_ERROR_UTF8
I'm only posting these for people who may be trying to understand why specific JSON files are not being decoded. Please do not hard-code these numbers into an error handler routine.

I used this simple script, flicked from StackOverflow to escape from the function failing:
<?php
 function utf8ize($d) {
 if (is_array($d)) {
 foreach ($d as $k => $v) {
 $d[$k] = utf8ize($v);
 }
 } else if (is_string ($d)) {
 return utf8_encode($d);
 }
 return $d;
 }
?>

Cheers,
Praveen Kumar!

when json_decode a empty string, PHP7 will trigger an Syntax error:
<?php
json_decode("");
var_dump(json_last_error(), json_last_error_msg());
// PHP 7
int(4)
string(12) "Syntax error"
// PHP 5
int(0)
string(8) "No error"

use this code with mb_convert_encoding, you can json_encode some corrupt UTF-8 chars
 function safe_json_encode($value, $options = 0, $depth = 512) {
 $encoded = json_encode($value, $options, $depth);
 if ($encoded === false && $value && json_last_error() == JSON_ERROR_UTF8) {
 $encoded = json_encode(utf8ize($value), $options, $depth);
 }
 return $encoded;
 }
 function utf8ize($mixed) {
 if (is_array($mixed)) {
 foreach ($mixed as $key => $value) {
 $mixed[$key] = utf8ize($value);
 }
 } elseif (is_string($mixed)) {
 return mb_convert_encoding($mixed, "UTF-8", "UTF-8");
 }
 return $mixed;
 }

Just adding this note since I had to code this for the actual values reference.
<?php
echo JSON_ERROR_NONE . ' JSON_ERROR_NONE' . '<br />';
echo JSON_ERROR_DEPTH . ' JSON_ERROR_DEPTH' . '<br />';
echo JSON_ERROR_STATE_MISMATCH . ' JSON_ERROR_STATE_MISMATCH' . '<br />';
echo JSON_ERROR_CTRL_CHAR . ' JSON_ERROR_CTRL_CHAR' . '<br />';
echo JSON_ERROR_SYNTAX . ' JSON_ERROR_SYNTAX' . '<br />';
echo JSON_ERROR_UTF8 . ' JSON_ERROR_UTF8' . '<br />';
echo JSON_ERROR_RECURSION . ' JSON_ERROR_RECURSION' . '<br />';
echo JSON_ERROR_INF_OR_NAN . ' JSON_ERROR_INF_OR_NAN' . '<br />';
echo JSON_ERROR_UNSUPPORTED_TYPE . ' JSON_ERROR_UNSUPPORTED_TYPE' . '<br />';
/*
The above outputs :
0 JSON_ERROR_NONE
1 JSON_ERROR_DEPTH
2 JSON_ERROR_STATE_MISMATCH
3 JSON_ERROR_CTRL_CHAR
4 JSON_ERROR_SYNTAX
5 JSON_ERROR_UTF8
6 JSON_ERROR_RECURSION
7 JSON_ERROR_INF_OR_NAN
8 JSON_ERROR_UNSUPPORTED_TYPE
*/
?>

This is a quite simple and functional trick to validate JSON's strings.
<?php
 function json_validate($string) {
 if (is_string($string)) {
 @json_decode($string);
 return (json_last_error() === JSON_ERROR_NONE);
 }
 return false;
 }
 echo (json_validate('{"test": "valid JSON"}') ? "It's a JSON" : "NOT is a JSON"); // prints 'It's a JSON'
 echo (json_validate('{test: valid JSON}') ? "It's a JSON" : "NOT is a JSON"); // prints 'NOT is a JSON' due to missing quotes
 echo (json_validate(array()) ? "It's a JSON" : "NOT is a JSON"); // prints 'NOT is a JSON' due to a non-string argument
?>

Cheers

Protected and private properties are ignored,
when json_encoding a class instance.
The snippet
<?php
class Example
{
 private $privateprop = "private property";
 protected $protectedprop = "protected property";
 public $publicprop = "public property";
}
echo json_encode(new Example);
?>

only returns 
{"publicprop":"public property"}

here is a small updated version of utf8ize that has the following addition : 
* It uses iconv instead of utf8_encode for potentially better result.
* It adds the support of objects variable 
* It also update array key value (in a case I met I had to utf8ize the key as well as those were generated from a user input value)
Here is the code.
<?php
 function utf8ize($d) {
 if (is_array($d)) {
 foreach ($d as $k => $v) {
 unset($d[$k]);
 $d[utf8ize($k)] = utf8ize($v);
 }
 } else if (is_object($d)) {
 $objVars = get_object_vars($d);
 foreach($objVars as $key => $value) {
 $d->$key = utf8ize($value);
 } 
 } else if (is_string ($d)) {
 return iconv('UTF-8', 'UTF-8//IGNORE', utf8_encode($d));
 } 
 return $d;
 }
?>