when does php json_decode return false?

Question

In the PHP documentation for json_decode it says it can return TRUE , FALSE , NULL .

Could some help me understand when it would return FALSE ? I understand invalid JSON will return NULL , but when would the other two be returned if not the actual JSON value?

Thanks

Answer 1

JSON format definition clearly shows all possible values and their representations:

A value can be a string in double quotes, or a number, or true or false or null , or an object or an array.

Both objects and arrays have special syntax in JSON representation (wrapped in {} and [] respectively), so they can't be mixed up with false in any case. The same goes with string - it's wrapped in "" (double quotation marks). As for Numbers, they have to contain at least one digit - so cannot be confused with false (and true and null ) too.

So that leaves us with the only case: when json_encode processes an object having redefined its JSON representation. For example (PHP 5.4+):

class FalsyFoo implements JsonSerializable {
  public $foo;

  public function __construct($f) {
    $this->foo = $f;
  }

  public function jsonSerialize() {  
    return false; 
  }
}

$f = new FalsyFoo(true);
$fj = json_encode($f);
var_dump( $fj ); // string(5) 'false'
var_dump( json_decode($fj) ); // bool(false)

Technically, we still work with false value here, but the source is obviously different.

---

If you're still not convinced, check the source code of json_decode , which calls php_json_decode_ex after checking the arguments. This, in turn, calls parse_JSON_ex first, which operates over the predefined state transition table ; the latter has only one set of states leading to false value as result. If this call fails somehow, value is checked directly :

if (str_len == 4) {
  if (!strcasecmp(str, "null")) {
    /* We need to explicitly clear the error 
         because its an actual NULL and not an error */
    jp->error_code = PHP_JSON_ERROR_NONE;
    RETVAL_NULL();
  } else if (!strcasecmp(str, "true")) {
    RETVAL_BOOL(1);
  }
} else if (str_len == 5 && !strcasecmp(str, "false")) {
  RETVAL_BOOL(0);
}

... and that's the only case when return_value is set to boolean.

Answer 2

The documentation says that values true, false and null (case-insensitive) are returned as TRUE, FALSE and NULL respectively. This means that if the booleans true or false are in the object to be encoded, they will be shows as TRUE or FALSE , and the same for null. For example:

json_decode('["hello",true]');

would return:

["hello",TRUE]

It doesn't mean that json_decode will return values of true , false , or null