Detect bad json data in PHP json_decode()?

Question

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 '10 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](http://json.org/) to find a good library. — whiskeysierra, Feb 27 '10 at 17:02

score 126 · Accepted Answer · answered Feb 27 '10 at 17:01

Here are a couple of things about json_decode :

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](https://stackoverflow.com/a/44448921/1327983), you need only check for `JSON_ERROR_NONE !== json_last_error()` — alttag, Apr 12 '19 at 02:13
PHP 8 returns the error if a function fails with "@" (instead of returning false as before). — migli, Jan 25 '23 at 07:23

dtar · Answer 2 · 2019-05-01T15:31:45.293

38

Since PHP 7.3, the json_decode function will accept a new JSON_THROW_ON_ERROR option that will let json_decode throw an exception instead of returning null on error.

Example:

try {  
  json_decode("{", false, 512, JSON_THROW_ON_ERROR);  
}  
catch (\JsonException $exception) {  
  echo $exception->getMessage(); // displays "Syntax error"  
}

edited May 01 '19 at 15:31

answered May 01 '19 at 14:58

dtar

1,469
12
17

5

This should be the accepted answer. [`JSON_THROW_ON_ERROR` constant](https://www.php.net/manual/en/json.constants.php) – A1rPun Sep 03 '19 at 20:48
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 ` – domdambrogia Apr 28 '20 at 01:35
1

@domdambrogia The JSON_THROW_ON_ERROR constant is only available from PHP 7.3 – David DIVERRES Nov 05 '20 at 15:25
doesn't work on 7.3.24 the error is there but couldn't catch it – TomSawyer Oct 10 '21 at 15:10

score 27 · Answer 3 · answered Jul 30 '13 at 15:31

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 http://php.net/manual/en/function.json-last-error-msg.php — vvondra, Mar 04 '15 at 00:16
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 03 '19 at 20:45

score 11 · Answer 4 · answered Jun 09 '17 at 03:41

This is how Guzzle handles json

    /**
 * Parse the JSON response body and return an array
 *
 * @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;
}

score 10 · Answer 5 · edited Oct 16 '18 at 08:55

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.

score 3 · Answer 6 · answered Sep 17 '19 at 16:26

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.

<?php
/**
 * 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

score 1 · Answer 7 · edited Oct 16 '18 at 08:54

/**
 * 
 * custom json_decode 
 * handle json_decode errors
 * 
 * @param type $json_text
 * @return type
 */
public static function custom_json_decode($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'
            );
    }
}

score 0 · Answer 8 · answered Feb 10 '23 at 17:20

For PHP 5.2+ . Note the first check.

<?php

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;
    }
}

Detect bad json data in PHP json_decode()?

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