Detect bad json data in PHP json_decode()?

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.
Learn more about Collectives
Teams
Q&A for work
Connect and share knowledge within a single location that is structured and easy to search.
Learn more about Teams
I'm trying handle bad json data when parsed through json_decode(). I'm using the following script:
if(!json_decode($_POST)) {
  echo "bad json data!";
  exit;
If $_POST equals:
'{ bar: "baz" }'
Then json_decode handles the error fine and spits out "bad json data!";
However, if I set $_POST to something like "invalid data", it gives me:
Warning: json_decode() expects parameter 1 to be string, array given in C:\server\www\myserver.dev\public_html\rivrUI\public_home\index.php  on line 6
bad json data!
Do I need to write a custom script to detect valid json data, or is there some other nifty way to detect this?
                $_POST is always an array containing the x-www-form-urlencoded parameters passed via POST. How do you send your data to your PHP script?
– Gumbo
                Feb 27, 2010 at 16:58
                The included json functions in PHP are not much help. They have a lot of issues. Take a look at json.org to find a good library.
– whiskeysierra
                Feb 27, 2010 at 17:02
it returns the data, or null when there is an error
it can also return null when there is no error : when the JSON string contains null
it raises a warning where there is a warning -- warning that you want to make disappear.
To solve the warning problem, a solution would be to use the @ operator (I don't often recommend using it, as it makes debuging a lot more harder... But here, there is not much of a choice) :
$_POST = array(
    'bad data'
$data = @json_decode($_POST);
You'd then have to test if $data is null -- and, to avoid the case in which json_decode returns null for null in the JSON string, you could check json_last_error, which (quoting) :
  Returns the last error (if any)
  occurred by last JSON parsing.
Which means you'd have to use some code like the following :
if ($data === null
    && json_last_error() !== JSON_ERROR_NONE) {
    echo "incorrect data";
                As Harry Bosch pointed out in another answer, you need only check for JSON_ERROR_NONE !== json_last_error()
– alttag
                Apr 12, 2019 at 2:13
                PHP 8 returns the error if a function fails with "@" (instead of returning false as before).
– migli
                Jan 25 at 7:23
catch (\JsonException $exception) {  
  echo $exception->getMessage(); // displays "Syntax error"  
                I received an exception: "Warning: Use of undefined constant JSON_THROW_ON_ERROR - assumed 'JSON_THROW_ON_ERROR' (this will throw an Error in a future version of PHP) in <file_path>
– domdambrogia
                Apr 28, 2020 at 1:35
You can also use json_last_error : http://php.net/manual/en/function.json-last-error.php
which as documentation says :   
  Returns the last error (if any) occurred during the last JSON
  encoding/decoding.
here is an example
json_decode($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;
                This is completely unnecessary in PHP 5.5 with php.net/manual/en/function.json-last-error-msg.php
– vvondra
                Mar 4, 2015 at 0:16
                You could do something like this with json_last_error_msg: if (JSON_ERROR_NONE !== json_last_error()) throw new Exception(json_last_error_msg());
– A1rPun
                Sep 3, 2019 at 20:45
 * @return array|string|int|bool|float
 * @throws RuntimeException if the response body is not in JSON format
public function json()
    $data = json_decode((string) $this->body, true);
    if (JSON_ERROR_NONE !== json_last_error()) {
        throw new RuntimeException('Unable to parse response body into JSON: ' . json_last_error());
    return $data === null ? array() : $data;
I just broke my head over a json syntax error in what appeared to be perfect json: {"test1":"car", "test2":"auto"} from a url encoded string.
But in my case some of the above was html encoded, as adding html_entity_decode($string) did the trick.
$ft = json_decode(html_entity_decode(urldecode(filter_input(INPUT_GET, 'ft', FILTER_SANITIZE_STRING))));
Hopefully this will save someone else some time.
I just spent an hour going through all the possible solutions on this page. I took the liberty of collective all these possible solutions into one function, to make it quicker and easier to try and debug. 
I hope it can be of use to someone else. 
 * Decontaminate text
 * Primary sources:
 *  - https://stackoverflow.com/questions/17219916/json-decode-returns-json-error-syntax-but-online-formatter-says-the-json-is-ok
 *  - https://stackoverflow.com/questions/2348152/detect-bad-json-data-in-php-json-decode
function decontaminate_text(
  $text,
  $remove_tags = true,
  $remove_line_breaks = true,
  $remove_BOM = true,
  $ensure_utf8_encoding = true,
  $ensure_quotes_are_properly_displayed = true,
  $decode_html_entities = true
  if ( '' != $text && is_string( $text ) ) {
    $text = preg_replace( '@<(script|style)[^>]*?>.*?</\\1>@si', '', $text );
    $text = str_replace(']]>', ']]&gt;', $text);
    if( $remove_tags ){
      // Which tags to allow (none!)
      // $text = strip_tags($text, '<p>,<strong>,<span>,<a>');
      $text = strip_tags($text, '');
    if( $remove_line_breaks ){
      $text = preg_replace('/[\r\n\t ]+/', ' ', $text);
      $text = trim( $text );
    if( $remove_BOM ){
      // Source: https://stackoverflow.com/a/31594983/1766219
      if( 0 === strpos( bin2hex( $text ), 'efbbbf' ) ){
        $text = substr( $text, 3 );
    if( $ensure_utf8_encoding ){
      // Check if UTF8-encoding
      if( utf8_encode( utf8_decode( $text ) ) != $text ){
        $text = mb_convert_encoding( $text, 'utf-8', 'utf-8' );
    if( $ensure_quotes_are_properly_displayed ){
      $text = str_replace('&quot;', '"', $text);
    if( $decode_html_entities ){
      $text = html_entity_decode( $text );
     * Other things to try
     * - the chr-function: https://stackoverflow.com/a/20845642/1766219
     * - stripslashes (THIS ONE BROKE MY JSON DECODING, AFTER IT STARTED WORKING, THOUGH): https://stackoverflow.com/a/28540745/1766219
     * - This (improved?) JSON-decoder didn't help me, but it sure looks fancy: https://stackoverflow.com/a/43694325/1766219
  return $text;
// Example use
$text = decontaminate_text( $text );
// $text = decontaminate_text( $text, false ); // Debug attempt 1
// $text = decontaminate_text( $text, false, false ); // Debug attempt 2
// $text = decontaminate_text( $text, false, false, false ); // Debug attempt 3
$decoded_text = json_decode( $text, true );
echo json_last_error_msg() . ' - ' . json_last_error();
I'll maintain it here: https://github.com/zethodderskov/decontaminate-text-in-php/blob/master/decontaminate-text-preparing-it-for-json-decode.php
public static function custom_json_decode($json_text) {
    $decoded_array = json_decode($json_text, TRUE);
    switch (json_last_error()) {
        case JSON_ERROR_NONE:
            return array(
                "status" => 0,
                "value" => $decoded_array
        case JSON_ERROR_DEPTH:
            return array(
                "status" => 1,
                "value" => 'Maximum stack depth exceeded'
        case JSON_ERROR_STATE_MISMATCH:
            return array(
                "status" => 1,
                "value" => 'Underflow or the modes mismatch'
        case JSON_ERROR_CTRL_CHAR:
            return array(
                "status" => 1,
                "value" => 'Unexpected control character found'
        case JSON_ERROR_SYNTAX:
            return array(
                "status" => 1,
                "value" => 'Syntax error, malformed JSON'
        case JSON_ERROR_UTF8:
            return array(
                "status" => 1,
                "value" => 'Malformed UTF-8 characters, possibly incorrectly encoded'
        default:
            return array(
                "status" => 1,
                "value" => 'Unknown error'
class JsonUtil {
    static function decode($json, $associative = null, $depth = null) {
        if (strtolower(trim($json)) === "null") {
            return null;
        if (is_null($associative)) {
            $res = json_decode($json);
        else {
            if (is_null($depth)) {
                $depth = 512;
            $res = json_decode($json, $associative, $depth);
        if (is_null($res)) {
            throw new Exception("Unable to decode JSON");
        return $res;
        Thanks for contributing an answer to Stack Overflow!
Please be sure to answer the question. Provide details and share your research!
But avoid …
Asking for help, clarification, or responding to other answers.
Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.