PHP ReflectionClass::isReadOnly() Method: Usage and Examples

When building tools that inspect PHP code, ReflectionClass::isReadOnly() helps answer a focused question: reports whether the class itself was declared readonly.

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

What Is ReflectionClass::isReadOnly()?

Reports whether the class itself was declared readonly.

Its method signature is:

public function isReadOnly(): bool

This method has no required arguments.

Return value: bool

Example 1: Inspect a PHP class declaration

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

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_CLASS_CONSTANT | Attribute::TARGET_METHOD | Attribute::TARGET_PROPERTY | Attribute::TARGET_PARAMETER | Attribute::TARGET_FUNCTION)]
class GuideTag { public function __construct(public string $label) {} }
trait HasIdentifier { public function identifier(): int { return 20; } }
interface NamedEntity { public function name(): string; }
class BaseEntity { public const string BASE = 'entity'; protected int $revision = 1; public static int $total = 4; }
class Shipment extends BaseEntity implements NamedEntity, IteratorAggregate {
    use HasIdentifier;
    #[GuideTag('category')]
    final public const string TYPE = 'parcel';
    private const int SECRET = 5;
    public string $title = 'Box';
    public function __construct(public int $number = 10) {}
    public function name(): string { return $this->title; }
    public function getIterator(): Traversable { yield 'item' => $this->title; }
    public function deliver(string $destination = 'office'): string { return 'Sent to ' . $destination; }
}
readonly class ImmutableShipment { public function __construct(public string $title = 'Box') {} }

$classReflection = new ReflectionClass(ImmutableShipment::class);

$result = $classReflection->isReadOnly();
var_dump($result);

Output:

bool(true)

Explanation: Reports whether the class itself was declared readonly.

Example 2: Compare a different class target

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

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_CLASS_CONSTANT | Attribute::TARGET_METHOD | Attribute::TARGET_PROPERTY | Attribute::TARGET_PARAMETER | Attribute::TARGET_FUNCTION)]
class GuideTag { public function __construct(public string $label) {} }
trait HasIdentifier { public function identifier(): int { return 20; } }
interface NamedEntity { public function name(): string; }
class BaseEntity { public const string BASE = 'entity'; protected int $revision = 1; public static int $total = 4; }
class Shipment extends BaseEntity implements NamedEntity, IteratorAggregate {
    use HasIdentifier;
    #[GuideTag('category')]
    final public const string TYPE = 'parcel';
    private const int SECRET = 5;
    public string $title = 'Box';
    public function __construct(public int $number = 10) {}
    public function name(): string { return $this->title; }
    public function getIterator(): Traversable { yield 'item' => $this->title; }
    public function deliver(string $destination = 'office'): string { return 'Sent to ' . $destination; }
}
readonly class ImmutableShipment { public function __construct(public string $title = 'Box') {} }

$classReflection = new ReflectionClass(Shipment::class);

$result = $classReflection->isReadOnly();
var_dump($result);

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

Output:

bool(false)

Result type: bool

Explanation: An object with readonly properties is not automatically a readonly class.

Practical Example: Apply reflection to a real object model

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

<?php

declare(strict_types=1);

#[Attribute(Attribute::TARGET_CLASS_CONSTANT | Attribute::TARGET_METHOD | Attribute::TARGET_PROPERTY | Attribute::TARGET_PARAMETER | Attribute::TARGET_FUNCTION)]
class GuideTag { public function __construct(public string $label) {} }
trait HasIdentifier { public function identifier(): int { return 20; } }
interface NamedEntity { public function name(): string; }
class BaseEntity { public const string BASE = 'entity'; protected int $revision = 1; public static int $total = 4; }
class Shipment extends BaseEntity implements NamedEntity, IteratorAggregate {
    use HasIdentifier;
    #[GuideTag('category')]
    final public const string TYPE = 'parcel';
    private const int SECRET = 5;
    public string $title = 'Box';
    public function __construct(public int $number = 10) {}
    public function name(): string { return $this->title; }
    public function getIterator(): Traversable { yield 'item' => $this->title; }
    public function deliver(string $destination = 'office'): string { return 'Sent to ' . $destination; }
}
readonly class ImmutableShipment { public function __construct(public string $title = 'Box') {} }

$classReflection = new ReflectionClass(ImmutableShipment::class);

$result = $classReflection->isReadOnly();
var_dump($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:

bool(true)

Status: yes

Explanation: ReflectionProperty::isReadOnly inspects one property instead.

Important Behavior and Edge Cases

• An object with readonly properties is not automatically a readonly class.
• ReflectionProperty::isReadOnly inspects one property instead.
• 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

• Reports whether the class itself was declared readonly.
• An object with readonly properties is not automatically a readonly class.
• ReflectionProperty::isReadOnly inspects one property instead.

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