How to validate JSON in PHP older than 5.3.0?

Question

$ob = json_decode($json);
if($ob === null) {
 // $ob is null because the json cannot be decoded
}

$data = json_decode($json_string);
if (is_null($data)) {
   die("Something dun gone blowed up!");
}

If you want to check if your input is valid JSON, you might as well be interested in validating whether or not it follows a specific format, i.e a schema. In this case you can define your schema using JSON Schema and validate it using this library.

Example:

person.json

{
    "title": "Person",
    "type": "object",
    "properties": {
        "firstName": {
            "type": "string"
        },
        "lastName": {
            "type": "string"
        },
        "age": {
            "description": "Age in years",
            "type": "integer",
            "minimum": 0
        }
    },
    "required": ["firstName", "lastName"]
}

Validation

<?php

$data = '{"firstName":"Hermeto","lastName":"Pascoal"}';

$validator = new JsonSchema\Validator;
$validator->validate($data, (object)['$ref' => 'file://' . realpath('person.json')]);

$validator->isValid()

Furthermore you can have a look on http://php.net/manual/en/function.json-last-error-msg.php that contain implementations of the missing function.

One of them is:

if (!function_exists('json_last_error_msg')) {
        function json_last_error_msg() {
            static $ERRORS = array(
                JSON_ERROR_NONE => 'No error',
                JSON_ERROR_DEPTH => 'Maximum stack depth exceeded',
                JSON_ERROR_STATE_MISMATCH => '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'
            );

            $error = json_last_error();
            return isset($ERRORS[$error]) ? $ERRORS[$error] : 'Unknown error';
        }
    }

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