PHP: rfc:scope-functions

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====== PHP RFC: Scope Functions ======

  * Version: 0.2
  * Date: 2026-04-26
  * Authors: Bob Weinand <bobwei9@hotmail.com>, Volker Dusch <edorian@php.net>
  * Status: Draft
  * Target: PHP 8.6
  * Implementation: Draft at https://github.com/php/php-src/compare/master...bwoebi:php-src:scope_fn

===== Introduction =====

This RFC proposes a variant of closures for the common case of a callback that runs //inside// its defining function.

Current PHP closures that are supposed to affect outside variables require explicit capturing:

<code php>
function () use (&$x, &$y, &$z) {
    // ...
}
</code>

The list is verbose, potentially unfamiliar to readers due to references, and silently wrong when forgotten as adding a new variable to the body without updating ''use()'' just shadows the variable.

For use cases involving the ''array_*'' functions, transaction wrappers, async callbacks, sorting, and others, having closure automatically its parents’ scope makes these functions shorter and easier to read, write, and maintain.

<code php>
$autoScoped = 1;
$closure = fn () {
    var_dump($autoScoped);
    $autoScoped = 2;
}

// int(1)
$closure();

// int(2)
var_dump($autoScoped);
</code>

===== Proposal =====

Introduce a new type of closure that shares variables with the enclosing scope.

==== Syntax ====

<code>
fn(parameter_list)[: return_type] { body }
</code>

Disambiguated from arrow functions by a ''{'' block vs the ''=>'' expression. The ''fn'' keyword is reused to avoid introducing a new keyword. ''fn(...) { ... }''.

==== Semantics ====

A scope function is a ''Closure'' whose variables are scoped to the parent function. All reads, writes, and newly introduced variables share the parent's scope:

<code php>
function example() {
    $x = 1;
    (fn() {
        $x++;
        $new = 'hi';
    })();
    // int(2), string(2) "hi"
    var_dump($x, $new);
}
</code>

Beyond shared variables, a scope function behaves like any other closure:

  * **Return:** ''return'' returns from the scope function only, not from the parent.
  * **Exceptions:** Exceptions propagate normally to the caller of the scope function, then up the stack.
  * **Call stack:** Calls to a scope function appear as their own frame in ''debug_backtrace()'' and stack traces.
  * **''$this'':** Inside of methods, ''$this'' inside the closure is the same as outside of it.
  * **''extract(), compact(), $$var'':** All scope modifing functions and constructs to work and modify the parent's scope.

===== Examples =====

==== Async callbacks ====

<code php>
// Current approach:
// - `use` for each callback
// - Results come back as an indexed array that the caller has to unpack.
function findSharedLikes($baseUrl, $userIdOne, $userIdTwo, $token, $client) {
    $promises[] = \Amp\async(function () use ($baseUrl, $client, $userIdOne, $token) {
        $req = $client->request("$baseUrl/api/user/$userIdOne/likes", ["Authorization" => "Bearer $token"]);
        return $req->getBody()->buffer();
    });
    $promises[] = \Amp\async(function () use ($baseUrl, $client, $userIdTwo, $token) {
        $req = $client->request("$baseUrl/api/user/$userIdTwo/likes", ["Authorization" => "Bearer $token"]);
        return $req->getBody()->buffer();
    });
    $results = \Amp\await($promises);
    return array_intersect($results[0], $results[1]);
}

// With scope functions:
// - results are written into nicely named variables in the parent scope
// - return is easy to read
function findSharedLikes($baseUrl, $userIdOne, $userIdTwo, $token, $client) {
    $promises[] = \Amp\async(fn() {
        $req = $client->request("$baseUrl/api/user/$userIdOne/likes", ["Authorization" => "Bearer $token"]);
        $likesOne = $req->getBody()->buffer();
    });
    $promises[] = \Amp\async(fn() {
        $req = $client->request("$baseUrl/api/user/$userIdTwo/likes", ["Authorization" => "Bearer $token"]);
        $likesTwo = $req->getBody()->buffer();
    });
    \Amp\await($promises);

    return array_intersect($likesOne, $likesTwo);
}
</code>

==== Transaction wrapper ====

A simple callback-based DB transaction wrapper that can be used with exceptions or return values.

<code php>
class DatabaseConnection
{
    const TRANSACTION_ABORT = 1;

    public function transaction(callable $callback): void {
        try {
            if ($callback() === self::TRANSACTION_ABORT) {
                $this->rollback();
                return;
            }
        } catch (\Throwable $e) {
            $this->rollback();
            throw $e;
        }
        $this->commit();
    }
}

$connection->transaction(fn () {
    // No need to capture $connection here or use $affectedRows by reference
    $affectedRows = $connection->query("UPDATE ...");
    if ($affectedRows === 0) {
        return DatabaseConnection::TRANSACTION_ABORT;
    }
    // ...
});
// $affectedRows is available after the transaction
log("Ran transaction, update affected $affectedRows rows");
</code>

==== Sorting with an external comparator ====

<code php>
function sortByPriority(array $items, PriorityTable $priorities, Metrics $metrics): array {
    // Track sort performance
    $comparisons = 0;

    // Today: $priorities could be used "normally" and $comparisons must be used by reference
    usort($items, function ($a, $b) use ($priorities, &$comparisons) {
        $comparisons++;
        return $priorities->of($a) <=> $priorities->of($b);
    });

    // With scope functions: Variables are simply in scope
    usort($items, fn($a, $b) {
        $comparisons++;
        return $priorities->of($a) <=> $priorities->of($b);
    });

    $metrics->recordComparisons($comparisons);
    return $items;
}
</code>

==== Accumulation ====

Aggregation of numbers into a total and count to avoid floating points errors in averages.

<code php>
function aggregate(array $numbers): array {
    $count = '0';
    $sum = '0';

    // Currently: Both accumulators must be listed in use() with &
    array_walk(function ($number) use (&$count, &$sum) {
        $count = bcadd($count, '1');
        $sum = bcadd($sum, $number);
    }, $numbers);

    // With scope functions: $count and $sum are just used.
    array_walk(fn($number) {
        $count = bcadd($count, '1')
        $sum = bcadd($sum, $number)
    )}, $numbers);

    return [$count, $sum];
}
</code>


==== Filtering with a side channel for errors ====

<code php>
function validateList(Validator $validator, array $items): array {
    $errors = [];

    // Today: $validator (read) and $errors (write) both have to be listed.
    $valid = array_filter($items, function ($item) use ($validator, &$errors) {
        if (!$reason = $validator->isValid($item)) {
            $errors[] = [$item, $reason];
            return false;
        }
        return true;
    });

    // With scope functions: nothing to list, intent reads top to bottom.
    $valid = array_filter($items, fn($item) {
        if (!$reason = $validator->isValid($item)) {
            $errors[] = [$item, $reason];
            return false;
        }
        return true;
    });

    return [$valid, $errors];
}
</code>

===== Restrictions =====

A scope function cannot leave the lifetime of its defining function.

The following forms are therefore not useful and thus forbidden:

  * **''static fn() {}'':** ''$this'' is already shared, a static scope function would be incoherent.
    <code>
Fatal error: Scope functions cannot be static in %s on line %d
</code>

  * **''fn() use () {}'':** Every variable is already shared. ''use()'' is redundant.
    <code>
Parse error: syntax error, unexpected token "use", expecting "{" in %s on line %d
</code>

  * **Recursion:** Recursive calls will raise an ''Error''.
    <code>
Fatal error: Uncaught Error: Cannot recursively call scope function in %s:%d
</code>

  * **Cloning:** ''clone $fn'' on a scope function raises an ''Error''. Two clones would share the same parent scope, interfering with each other's state.

  * **Rebinding:** ''Closure::bind()'' and ''Closure::bindTo()'' raise an ''Error''. The scope and ''$this'' come from the parent and cannot be rebound.

  * **Outliving the parent:** A scope function may only be called while its defining function is still on the stack. Storing one in a property, a global, a static variable, or returning it from the parent raises an ''Error'' when the parent returns.
    <code>
Fatal error: Uncaught Error: Scope function closure must not outlive the declaring scope in %s:%d
</code>

  * **Re-evaluation in the same frame:** Re-evaluating the same scope function declaration in the same parent frame invalidates the previous instance rather than silently retargeting it to the new one.
    <code>
Fatal error: Uncaught Error: Cannot call scope function: defining scope has exited in %s:%d
</code>
    This avoids silently retargeting an old closure handle to a new instance.

  * **Generators:** There are no restrictions on yield/yield from. Generators will be closed as the scoped function goes out of scope.

===== Alternatives Considered =====

Letting an older closure handle transparently resolve to the current instance when the same scope function declaration is re-evaluated:

This was rejected because it makes closure identity unstable: A stored closure would silently change meaning after later execution of the parent frame.

===== Reflection =====

<code php>
ReflectionFunctionAbstract::isScopeFunction(): bool
</code>

Returns ''true'' for scope function closures. Available on ''ReflectionFunction''.

===== Backward Incompatible Changes =====

None. ''fn(...) { ... }'' is currently a parse error.

===== Open Issues =====

None.

===== Future Scope =====

Scoped functions can be inlined with their callers. For example, for ''array_map'' this is a trivial operation that will improve performance.

===== Proposed PHP Version =====

PHP 8.6.

===== Voting =====

Accept the scope functions feature as described? Yes/No. Requires 2/3 majority.

===== References =====

  * [[https://wiki.php.net/rfc/arrow_functions_v2|Arrow Functions 2.0 RFC]]
  * [[https://wiki.php.net/rfc/closures|Original Closures RFC]]
  * [[https://wiki.php.net/rfc/closure-optimizations|Closure optimizations]]
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