symfony/var-dumper · error · BadMethodCallException

objects are immutable.

Error message

objects are immutable.

What it means

Data objects are read-only views over cloned dump values. offsetSet() (invoked by ArrayAccess writes like $data[$key] = $value) unconditionally throws BadMethodCallException because Data is intentionally immutable.

Solutions

1. Do not mutate Data; convert first with $data->getValue() or iterator_to_array($data) and modify the plain array
2. Clone the raw value via $data->getValue(), change it, and re-dump if needed
3. If you need a mutable structure, build your own array from the Data's contents

Example fix

// before
$data['secret'] = '[redacted]';
// after
$arr = $data->getValue();
$arr['secret'] = '[redacted]';

if ($data instanceof \Symfony\Component\VarDumper\Cloner\Data) {
    // never mutate; copy out first
    $copy = is_array($v = $data->getValue()) ? $v : (array) $v;
}

function mutableCopy(\Symfony\Component\VarDumper\Cloner\Data $data): array {
    return is_array($v = $data->getValue()) ? $v : (array) $v;
}

try {
    $data[$key] = $value;
} catch (\BadMethodCallException $e) {
    // Data is immutable: work on a copied array instead
    $arr = $data->getValue();
}

When it happens

Trigger: Attempting to mutate a Data object with array-write syntax, e.g. `$data['key'] = 'x'` or `$data[] = $value`, including accidental mutation inside loops.

Common situations: Developers treating a dump/clone Data handle like a normal array and trying to modify it; mutating dump results to redact values before display.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• " " object holds non-iterable type " ".

AI-assisted analysis of symfony/var-dumper@e9d9cf5dcd (2026-09-14). Data as JSON: /api/errors/1da9b51ffdd3d18a. Report an issue: GitHub.

Appendix: source

Thrown at Cloner/Data.php:152

    public function __isset(string $key): bool
    {
        return null !== $this->seek($key);
    }

    public function offsetExists(mixed $key): bool
    {
        return $this->__isset($key);
    }

    public function offsetGet(mixed $key): mixed
    {
        return $this->__get($key);
    }

    public function offsetSet(mixed $key, mixed $value): void
    {
        throw new \BadMethodCallException(self::class.' objects are immutable.');
    }

    public function offsetUnset(mixed $key): void
    {
        throw new \BadMethodCallException(self::class.' objects are immutable.');
    }

    public function __toString(): string
    {
        $value = $this->getValue();

        if (!\is_array($value)) {
            return (string) $value;
        }

        return \sprintf('%s (count=%d)', $this->getType(), \count($value));
    }

View on GitHub (pinned to e9d9cf5dcd)