Casting types

Here we talk about casting types — how to make type inferring more precise.

Casting primitives with PHP operators

PHP has language constructs (int) and so on. KPHP uses them for compile-time inferring and runtime casting.

A common intention is to cast strings to integers and vice versa:

$a = isset($input['v']) ? (int)$input['v'] : 0;
$id = (int)"42";
$id_str = (string)$id;

Casts are pretty useful to narrow mixed to a presice primitive if we know exactly, what value it holds at runtime:

function getUser($id) {   // returns mixed 
  if($id < 0) return "invalid id=$id";
  return ['id' => $id, ...];

$user = getUser(1);         // mixed
// but we are SURE $id is correct, it's array really
$user = (array)getUser(1);  // mixed[]

Without casts, you can't pass mixed to strictly typed functions:

function getFriends(int $user_id) { /* ... */ }

$user = ['id' => 1, 'name' => "Alice"];   // mixed[]
getFriends($user['id']);                  // error! passing mixed to int
// but we are SURE that 'id' index is int at runtime
getFriends((int)$user['id']);             // ok

Casts can be used to drop false / null from primitives:

$s_n = something() ? "hello" : null;   // string|null
// again, if we are SURE it's really not null
$s = (string)$s_n;                     // string

Using (array) cast is a bit tricky: (array)mixed == mixed[], (array)(T[]|false|null) = T[].

function get($single) { return $single ? 1 : [1]; }   // returns mixed
$arr = (array)get(false);                             // mixed[]

$arr_or_null = something ? [1,2,3] : null;     // ?int[]
$arr = (array)$arr_or_null;                    // int[]

Casting instances

When you get an object of a base class, you can do instanceof check for derived. It works exactly like in PHP.
Inside if ($x instanceof A), $x is assumed to be of class A and should be read-only:

class Base { public $b; }
class Derived extends Base { public $d; }

function analyze(Base $o) {
  if ($o instanceof Derived) {
    $o->d;    // ok

If you are whyever sure, that $o is Derived without instanceof, there is instance_cast() operator:

/** @var Base[] $arr */
$d = instance_cast($arr[0], Derived::class);   // returns ?Derived (null if not of that class)
$d->d;    // ok if not null

Casting instances to mixed

Instances are not compatible with mixed, but…

You can deeply convert an object to mixed[] with instance_to_array().
It is useful for logging/debugging, as instances are not allowed to be passed to var_dump(), etc:

class Message {
  public $id;
  public $text;
  /** @var User */
  public $author;   // this field will be also converted (deep conversion is performed)
$message_as_array = instance_to_array($message);   // mixed[]

As a consequence, you can output an instance to JSON (KPHP has no native JSON serialization for instances yet):


Why instance_to_array($obj), but not native PHP (array)$obj? Because PHP’s (array)$obj is not suitable:

  • remains nested instances — not converted to arrays, not compatible with mixed[]
  • private and protected fields names are mangled with unprintable symbols

That’s why instance_to_array() — natively in KPHP and polyfilled in PHP — to guarantee identical behavior.

Smart casts

If you have already checked for is_array($v), you don't need explicit (array)$v: KPHP will infer it automatically.


function f_int(int $i) { /* ... */ }
function demo(?int $i_or_null) {
  if ($i_or_null !== null)

One more example:

 * @param tuple(int, A)|false $t
function demo($t) {
  if ($t === false) 

One more example:

/** @var (int|false|null)[] $arr */
foreach ($arr as $v) {
  if ($v === false || $v === null) 

Even one more example!

/** @var int|false $b */
if ($b) 
  f_int($b);   // KPHP understands, that false can't be here

Smart casts work only for local vars. They don't work for array indexing or property access.

if ($a->int_or_false !== false) {
  // won't compile! can't pass 'int|false' to 'int'

Why? Because there can be another reference to $a, which can potentially change values bypassing control flow.

Similarly, smart casts don't work when variables are passed by reference.

function changeVar(int &$ref) {}

/** @var ?int $a */
if ($a !== null) {
  // won't compile! can't pass '?int' to 'int'

Also, smart casts don't work on re-assigning:

function demo(?int $id) {
  if ($id === null) 
    $id = id();
  // error! $id here is still int|null

Don't confuse smart casts and lateinit pattern. Here is lateinit, not smart casts:

class A {
  /** @var ?array */
  static private $cached_info = null;
  static function getInfo(): array {
    if (self::$cached_info === null) 
      self::$cached_info = loadInfo();

    return self::$cached_info;     // error! returning ?array instead of array
    // correct: (array)... or not_null(...)

not_null, not_false

These keywords tell KPHP, that you guarantee a variable to carry a non-empty value. Similar to operator !! in Kotlin.

// correcting upper example
function getInfo(): array {
  if (self::$cached_info === null) 
    self::$cached_info = loadInfo();

  return not_null(self::$cached_info);

Use not_null() and not_false() when it is uninferrable or smart casts don't work, like above. It's much more convenient than (casts) and much more semantic.

Calling native functions does automatic casts

Say, you call strlen() function. It accepts a string argument. But if you call it with integer — it will be automatically cast. If you call it with mixed — the same.

function my_strlen(string $s) {}

my_strlen(4);              // error
strlen(4);                 // ok, automatically casted
my_strlen($user['name']);  // error
strlen($user['name']);     // ok, automatically casted 

No matter whether a native function accepts strings / ints / arrays — casts will be auto inserted.

Why is it done in such a way? Almost because when executed in plain PHP, standard library functions check input arguments, and if you pass something wrong — you'll see it while development, as opposed to functions written in PHP with auto inferred types. Moreover, such an approach requires less manual casts when using mixed, though it seems a bit incorrectly. Maybe, this behavior will be controlled by an option in the future.

When declare(strict_types=1) is used, implicit casting is disabled for that file. This applies to both native functions and functions annotated with @kphp-infer cast.