First class callable syntax

    The first class callable syntax is introduced as of PHP 8.1.0, as a way of creating anonymous functions from callable. It supersedes existing callable syntax using strings and arrays. The advantage of this syntax is that it is accessible to static analysis, and uses the scope at the point where the callable is acquired.

    CallableExpr(...) syntax is used to create a Closure object from callable. CallableExpr accepts any expression that can be directly called in the PHP grammar:

    Example #1 Simple first class callable syntax

    <?php

    class Foo {
    public function     method() {}
    public static function     staticmethod() {}
    public function     __invoke() {}
    }

    $obj = new Foo();
    $classStr = 'Foo';
    $methodStr = 'method';
    $staticmethodStr = 'staticmethod';


    $f1 = strlen(...);
    $f2 = $obj(...); // invokable object
    $f3 = $obj->method(...);
    $f4 = $obj->$methodStr(...);
    $f5 = Foo::staticmethod(...);
    $f6 = $classStr::$staticmethodStr(...);

    // traditional callable using string, array
    $f7 = 'strlen'(...);
    $f8 = [$obj, 'method'](...);
    $f9 = [Foo::class, 'staticmethod'](...);
    ?>

    Note:

    The ... is part of the syntax, and not an omission.

    CallableExpr(...) has the same semantics as Closure::fromCallable(). That is, unlike callable using strings and arrays, CallableExpr(...) respects the scope at the point where it is created:

    Example #2 Scope comparison of CallableExpr(...) and traditional callable

    <?php

    class Foo {
    public function     getPrivateMethod() {
    return [    $this, 'privateMethod'];
    }

    private function     privateMethod() {
    echo     __METHOD__, "\n";
    }
    }

    $foo = new Foo;
    $privateMethod = $foo->getPrivateMethod();
    $privateMethod();
    // Fatal error: Call to private method Foo::privateMethod() from global scope
    // This is because call is performed outside from Foo and visibility will be checked from this point.

    class Foo1 {
    public function     getPrivateMethod() {
    // Uses the scope where the callable is acquired.
    return $this->privateMethod(...); // identical to Closure::fromCallable([$this, 'privateMethod']);
    }

    private function     privateMethod() {
    echo     __METHOD__, "\n";
    }
    }

    $foo1 = new Foo1;
    $privateMethod = $foo1->getPrivateMethod();
    $privateMethod(); // Foo1::privateMethod
    ?>

    Note:

    Object creation by this syntax (e.g new Foo(...)) is not supported, because new Foo() syntax is not considered a call.

    Note:

    The first-class callable syntax cannot be combined with the nullsafe operator. Both of the following result in a compile-time error:

    <?php
    $obj    ?->method(...);
    $obj?->prop->method(...);
    ?>

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    User Contributed Notes 2 notes

    up
    14
    bienvenunet at yahoo dot com
    1 year ago
    There's a major gotcha with this syntax that may not be apparent until you use this syntax and find you're getting "Cannot rebind scope of closure created from method" exceptions in some random library code.

    As the documentation indicates, the first-class callable uses the scope at the point where the callable is acquired. This is fine as long as nothing in your code will attempt to bind the callable with the \Closure::bindTo method.

    I found this the hard way by changing callables going to Laravel's Macroable functionality from the array style to the first-class callable style. The Macroable functionality \Closure::bindTo calls on the callable.

    AFAIK, the only workaround is to use the uglier array syntax.
    up
    0
    koalay at gmail dot com
    9 days ago
    As of PHP 8.2, the first class callable syntax of a class method cannot be serialized like the old array callables.

    For example,

    <?php

    class Bird {
    public function     fly() {
    }
    }

    $bird = new Bird();
    $callable1 = [$bird, 'fly'];
    $callable2 = $bird->fly(...);

    // This is fine
    serialize($callable1);

    // This will cause a fatal error
    // "PHP Fatal error: Uncaught Exception: Serialization of 'Closure' is not allowed"
    serialize($callable2);

    ?>

    So if your callable is used across session and need to be serialized, you should not use the first class callable syntax (until this is somehow fixed in the future).
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