How to Reliably Detect Malformed JSON Data in PHP Using `json_decode()`?

How to Detect Malformed JSON Data Using JSON Decode in PHP

When attempting to parse JSON data using json_decode(), handling invalid formats is crucial to ensure data integrity. While this function is generally effective, it may exhibit inconsistencies when handling certain input types.

Problem Scenario

Consider the following scenario:

if(!json_decode($_POST)) {
  echo "bad json data!";
  exit;
}

This script aims to detect and handle invalid JSON data. However, when $_POST contains a valid JSON format (e.g., { bar: "baz" }), json_decode() manages the error correctly, displaying "bad json data!". However, when $_POST is set to an invalid format (e.g., "invalid data"), an unexpected warning is raised, followed by "bad json data!":

Warning: json_decode() expects parameter 1 to be string, array given in ...
bad json data!

Custom Detection or Alternative Solution?

To address this issue, developers may consider writing a custom script to detect valid JSON data or explore other available options.

Solution Using JSON Decode Features:

JSON decode has several key characteristics to consider:

• It returns the parsed data or null in case of an error.
• It may also return null for a valid case where the JSON string contains null.
• It raises warnings for certain errors.

To prevent the warning issue, the @ operator can be employed (although its use is discouraged due to potential debugging difficulties). This operator suppresses the warning while still allowing the script to continue execution.

$_POST = array('bad data');
$data = @json_decode($_POST);

After suppressing the warning, it is necessary to check whether $data is null. To differentiate between an invalid format and a valid null value, json_last_error() can be utilized. This function returns the most recent JSON parsing error, if any.

if ($data === null && json_last_error() !== JSON_ERROR_NONE) {
    echo "incorrect data";
}

By combining these techniques, we can effectively detect and handle invalid JSON data in PHP using json_decode().

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