How to determine whether a string is valid JSON?

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.

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

With PHP >= 5.3

// PHP >= 5.3function 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)";}

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