PHP 7.3: A Look at JSON Error Handling

One of the new features coming to PHP 7.3 is better error handling for json_encode() and json_decode(). The RFC was unanimously accepted by a 23 to 0 vote. Let’s take a look at how we handle JSON errors in <= PHP 7.2, and the new improvements coming in PHP 7.3.

#Background

In the current stable, PHP v7.2, if you want to determine if JSON is invalid, you have to use the json_last_error() function to verify:

>>> json_decode("{");
=> null
>>> json_last_error();
=> 4
>>> json_last_error() === JSON_ERROR_NONE
=> false
>>> json_last_error_msg()
=> "Syntax error"

For example, in the Laravel Illuminate\Encryption\Encrypter class here’s a check to make sure calling json_encode() doesn’t result in an error:

// Once we get the encrypted value we'll go ahead and base64_encode the input
// vector and create the MAC for the encrypted value so we can then verify
// its authenticity. Then, we'll JSON the data into the "payload" array.
$json = json_encode(compact('iv', 'value', 'mac'));
 
if (json_last_error() !== JSON_ERROR_NONE) {
    throw new EncryptException('Could not encrypt the data.');
}
 
return base64_encode($json);

We can at least determine if JSON encoding/decoding had an error, but it’s a little clunky compared to throwing an exception, which packages the error code and message neatly together.

Although you have to opt-in, v7.3 has an excellent way of allowing you to catch and handle JSON exceptions—let’s check out the new flag we can use!

#The Throw on Error Flag in PHP 7.3

With the new option flag JSON_THROW_ON_ERROR it’s possible to rewrite this block of code with a try/catch. Maybe some something like the following:

use JsonException;
 
try {
    $json = json_encode(compact('iv', 'value', 'mac'), JSON_THROW_ON_ERROR);
    return base64_encode($json);
} catch (JsonException $e) {
    throw new EncryptException('Could not encrypt the data.', 0, $e);
}

I think this new style is especially useful for userland code when you receive some JSON data and instead of digging around for json_last_error() and the matching flag, your JSON encoding and decoding can take advantage of the error handler.

The json_decode() function has a few more arguments, and will look something like the following in PHP 7.3 if you want to take advantage of error handling:

use JsonException;
 
try {
    return json_decode($jsonString, $assoc = true, $depth = 512, JSON_THROW_ON_ERROR);
} catch (JsonException $e) {
    // Handle the JSON Exception
}
 
// Or even just let it bubble up...
 
/**
 * Decode a JSON string into an array
 *
 * @return array
 * @throws JsonException
 */
function decode($jsonString) {
    return json_decode($jsonString, $assoc = true, $depth = 512, JSON_THROW_ON_ERROR);
}

#Getting the Error Code and Message

Previously you retrieve the JSON error code and message using the following functions:

// Error code
json_last_error();
 
// Human-friendly message
json_last_error_msg();

If you use the new JSON_THROW_ON_ERROR flag, here’s how you’ll get the code and message:

try {
    return json_decode($jsonString, $assoc = true, $depth = 512, JSON_THROW_ON_ERROR);
} catch (JsonException $e) {
    $e->getMessage(); // like json_last_error_msg()
    $e->getCode(); // like json_last_error()
}

See the base Exception class for more API details, the JsonException exception is a subclass of Exception.

#JSON’s Default Behavior in PHP 7.3

When upgrading to PHP 7.3, your code will be backward compatible on day one and continue to work as expected.

PHP’s default json_encode|decode() behavior hasn’t changed, the throw on error RFC adds a new option and exception class.

#Learn More

You can learn all about the most prominent features coming to PHP 7.3 in our post announcing the PHP 7.3 Alpha 1 release last week. Also, read through the RFC for JSON_THROW_ON_ERROR for full details on the proposal coming to PHP 7.3.