(PHP 5 >= 5.3.0, PHP 7)
json_last_error — Returns the last error occurred
Returns the last error (if any) occurred during the last JSON encoding/decoding.
This function has no parameters.
Returns an integer, the value can be one of the following constants:
| Constant | Meaning | Availability | 
|---|---|---|
| JSON_ERROR_NONE | No error has occurred | |
| JSON_ERROR_DEPTH | The maximum stack depth has been exceeded | |
| JSON_ERROR_STATE_MISMATCH | Invalid or malformed JSON | |
| JSON_ERROR_CTRL_CHAR | Control character error, possibly incorrectly encoded | |
| JSON_ERROR_SYNTAX | Syntax error | |
| JSON_ERROR_UTF8 | Malformed UTF-8 characters, possibly incorrectly encoded | PHP 5.3.3 | 
| JSON_ERROR_RECURSION | One or more recursive references in the value to be encoded | PHP 5.5.0 | 
| JSON_ERROR_INF_OR_NAN | One or more NANorINFvalues in the value to be encoded | PHP 5.5.0 | 
| JSON_ERROR_UNSUPPORTED_TYPE | A value of a type that cannot be encoded was given | PHP 5.5.0 | 
Example #1 json_last_error() example
<?php
// A valid json string
$json[] = '{"Organization": "PHP Documentation Team"}';
// An invalid json string which will cause an syntax 
// error, in this case we used ' instead of " for quotation
$json[] = "{'Organization': 'PHP Documentation Team'}";
foreach ($json as $string) {
    echo 'Decoding: ' . $string;
    json_decode($string);
    switch (json_last_error()) {
        case JSON_ERROR_NONE:
            echo ' - No errors';
        break;
        case JSON_ERROR_DEPTH:
            echo ' - Maximum stack depth exceeded';
        break;
        case JSON_ERROR_STATE_MISMATCH:
            echo ' - Underflow or the modes mismatch';
        break;
        case JSON_ERROR_CTRL_CHAR:
            echo ' - Unexpected control character found';
        break;
        case JSON_ERROR_SYNTAX:
            echo ' - Syntax error, malformed JSON';
        break;
        case JSON_ERROR_UTF8:
            echo ' - Malformed UTF-8 characters, possibly incorrectly encoded';
        break;
        default:
            echo ' - Unknown error';
        break;
    }
    echo PHP_EOL;
}
?>
The above example will output:
Decoding: {"Organization": "PHP Documentation Team"} - No errors
Decoding: {'Organization': 'PHP Documentation Team'} - Syntax error, malformed JSON
Example #2 json_last_error() with json_encode()
<?php
// An invalid UTF8 sequence
$text = "\xB1\x31";
$json  = json_encode($text);
$error = json_last_error();
var_dump($json, $error === JSON_ERROR_UTF8);
?>
The above example will output:
string(4) "null" bool(true)