How to determine whether a string is valid JSON?

Question

Does anyone know of a robust (and bullet proof) is_JSON function snippet for PHP? I (obviously) have a situation where I need to know if a string is JSON or not.

Hmm, perhaps run it through a JSONLint request/response, but that seems a bit overkill.

Answer 1

$this->post_data = json_decode( stripslashes( $post_data ) );
  if( $this->post_data === NULL )
   {
   die( '{"status":false,"msg":"The post_data parameter must be valid JSON"}' );
   }

Answer 2

For my projects I use this function (please read the "Note" on the json_decode() docs).

Passing the same arguments you would pass to json_decode() you can detect specific application "errors" (e.g. depth errors)

With PHP >= 5.6

// PHP >= 5.6
function is_JSON(...$args) {
    json_decode(...$args);
    return (json_last_error()===JSON_ERROR_NONE);
}

With PHP >= 5.3

// PHP >= 5.3
function is_JSON() {
    call_user_func_array('json_decode',func_get_args());
    return (json_last_error()===JSON_ERROR_NONE);
}

Usage example:

$mystring = '{"param":"value"}';
if (is_JSON($mystring)) {
    echo "Valid JSON string";
} else {
    $error = json_last_error_msg();
    echo "Not valid JSON string ($error)";
}

Answer 3

What about using json_decode, which should return null if the given string was not valid JSON-encoded data ?

See example 3 on the manual page :

// the following strings are valid JavaScript but not valid JSON

// the name and value must be enclosed in double quotes
// single quotes are not valid 
$bad_json = "{ 'bar': 'baz' }";
json_decode($bad_json); // null

// the name must be enclosed in double quotes
$bad_json = '{ bar: "baz" }';
json_decode($bad_json); // null

// trailing commas are not allowed
$bad_json = '{ bar: "baz", }';
json_decode($bad_json); // null

Answer 4

If you are using the built in json_decode PHP function, json_last_error returns the last error (e.g. JSON_ERROR_SYNTAX when your string wasn't JSON).

Usually json_decode returns null anyway.

Answer 5

Doesn't json_decode() with a json_last_error() work for you? Are you looking for just a method to say "does this look like JSON" or actually validate it? json_decode() would be the only way to effectively validate it within PHP.