PHP ReflectionFunctionAbstract::getClosureUsedVariables() Method: Usage and Examples

When building tools that inspect PHP code, ReflectionFunctionAbstract::getClosureUsedVariables() helps answer a focused question: returns lexical variables captured by a closure with use.

Reflection is useful in framework tooling, dependency injection, testing, code inspection and debugging.

What Is ReflectionFunctionAbstract::getClosureUsedVariables()?

Returns lexical variables captured by a closure with use.

Its method signature is:

public function getClosureUsedVariables(): array

This method has no required arguments.

Return value: array

Example 1: Inspect a function or callable

Start with a self-contained PHP declaration and inspect the method result.

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_FUNCTION | Attribute::TARGET_METHOD | Attribute::TARGET_PARAMETER)]
class FunctionMarker { public function __construct(public string $label) {} }
/** Formats one price for a product listing. */
#[FunctionMarker('currency')]
function formatPrice(float $amount, string $currency = 'USD'): string {
    return $currency . ' ' . number_format($amount, 2);
}
function logMessages(string ...$messages): string { return implode(', ', $messages); }
function &returnReference(array &$values): array { return $values; }
function valuesFromGenerator(): Generator { yield 10; yield 20; }
class FormatterService {
    public function label(int $id): string { return '#' . $id; }
    public static function uppercase(string $message): string { return strtoupper($message); }
}
$prefix = "ID:";
$closure = static function (int $number) use ($prefix): string { static $calls = 0; $calls++; return $prefix . $number; };
$functionReflection = new ReflectionFunction($closure);

$result = $functionReflection->getClosureUsedVariables();
print_r($result);

Output:

Array
(
    [prefix] => ID:
)

Explanation: Returns lexical variables captured by a closure with use.

Example 2: Compare a different callable

Next, change the reflected declaration or parameter to observe a related situation.

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_FUNCTION | Attribute::TARGET_METHOD | Attribute::TARGET_PARAMETER)]
class FunctionMarker { public function __construct(public string $label) {} }
/** Formats one price for a product listing. */
#[FunctionMarker('currency')]
function formatPrice(float $amount, string $currency = 'USD'): string {
    return $currency . ' ' . number_format($amount, 2);
}
function logMessages(string ...$messages): string { return implode(', ', $messages); }
function &returnReference(array &$values): array { return $values; }
function valuesFromGenerator(): Generator { yield 10; yield 20; }
class FormatterService {
    public function label(int $id): string { return '#' . $id; }
    public static function uppercase(string $message): string { return strtoupper($message); }
}
$closure = static fn(int $number): int => $number + 1;
$functionReflection = new ReflectionFunction($closure);

$result = $functionReflection->getClosureUsedVariables();
print_r($result);

// Check the returned value type before using the result.
echo "\nResult type: " . get_debug_type($result);

Output:

Array
(
)

Result type: array

Explanation: Captures differ between arrow functions and explicit use expressions.

Practical Example: Use reflection in validation or tooling

This final case shows how the same API can be used when inspecting application definitions.

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_FUNCTION | Attribute::TARGET_METHOD | Attribute::TARGET_PARAMETER)]
class FunctionMarker { public function __construct(public string $label) {} }
/** Formats one price for a product listing. */
#[FunctionMarker('currency')]
function formatPrice(float $amount, string $currency = 'USD'): string {
    return $currency . ' ' . number_format($amount, 2);
}
function logMessages(string ...$messages): string { return implode(', ', $messages); }
function &returnReference(array &$values): array { return $values; }
function valuesFromGenerator(): Generator { yield 10; yield 20; }
class FormatterService {
    public function label(int $id): string { return '#' . $id; }
    public static function uppercase(string $message): string { return strtoupper($message); }
}
$prefix = "ORDER:";
$closure = static function (int $number) use ($prefix): string { static $calls = 0; $calls++; return $prefix . $number; };
$functionReflection = new ReflectionFunction($closure);

$result = $functionReflection->getClosureUsedVariables();
print_r($result);

// Use the reflection result in a simple decision.
if (is_bool($result)) {
    echo "\nStatus: " . ($result ? "yes" : "no");
} elseif (is_array($result)) {
    echo "\nEntries: " . count($result);
} elseif ($result === null) {
    echo "\nNo value returned";
} else {
    echo "\nReflection result available";
}

Output:

Array
(
    [prefix] => ORDER:
)

Entries: 1

Explanation: getStaticVariables includes static function variables too.

Important Behavior and Edge Cases

• Captures differ between arrow functions and explicit use expressions.
• getStaticVariables includes static function variables too.
• Reflection describes declarations and runtime state; requesting metadata does not automatically validate or sanitize application input.

Comparison With Related Approaches

Common Beginner Mistakes

Key Takeaways

• Returns lexical variables captured by a closure with use.
• Captures differ between arrow functions and explicit use expressions.
• getStaticVariables includes static function variables too.

Use ReflectionFunctionAbstract::getClosureUsedVariables() when you need the precise metadata or behavior demonstrated above. Keep the target declaration and relevant PHP version in mind when adapting the examples.