What causes "Constant "%s" is undefined." in twigphp/Twig?

Correct the constant name/namespace in the template to match the actual defined constant

twigphp/Twig · error · RuntimeError

Constant " " is undefined.

Error message

Constant "%s" is undefined.

What it means

CoreExtension::constant() throws this RuntimeError when Twig's `constant()` function is called with a constant name that is not defined via defined(), and the name is not the special `::class` case. This mirrors PHP's own "Undefined constant" error but as a Twig RuntimeError, so templates fail with a clear message instead of a PHP fatal Error.

Solutions

Correct the constant name/namespace in the template to match the actual defined constant
Verify with php -r 'var_dump(defined("FOO_BAR"));' or grep the class for the constant
Guard in the template with `{% if constant('FOO_BAR') is defined %}`... wait — use `{{ FOO_BAR is defined ? constant('FOO_BAR') : default }}` pattern via defined checks where supported
If the constant was removed upstream, replace it with the new name or inline the value

Example fix

// before (Twig template)
{{ constant('User::ROLE_ADMIN') }}
// after (constant lives on App\Entity\User and an object is in scope)
{{ constant('ROLE_ADMIN', user) }}

Defensive patterns

Strategy: validation

Validate before calling

// PHP, before rendering
if (!\defined('App\\Constants::ROLE_ADMIN')) {
    throw new \LogicException('Referenced constant no longer exists; update templates.');
}

Type guard

function constantExists(string $name, ?object $object = null): bool
{
    return null !== $object
        ? ('class' === $name || \defined($object::class.'::'.$name))
        : \defined($name);
}

Try / catch

try {
    $value = \Twig\Extension\CoreExtension::constant($name, $object);
} catch (\Twig\Error\RuntimeError $e) {
    if (!str_starts_with($e->getMessage(), 'Constant "') ) { throw $e; }
    $value = $default;
}

Prevention

Keep a central PHP constants class and reference its values in templates only via tested names
Run template smoke tests in CI that render pages using constant()
Grep templates when upgrading dependencies that may rename/remove constants
Use `is defined` checks in templates for optional constants

When it happens

Trigger: `{{ constant('FOO_BAR') }}` when FOO_BAR is not defined; `{{ constant('MISSING', object) }}` when the object's class lacks that constant; typos in constant names; constants removed from a dependency after a version upgrade; referencing constants with wrong casing/namespace in the template string.

Common situations: Upgrading a library that removed or renamed a class constant while templates still reference it; typo in the constant name inside the template; the constant exists only in a different namespace than the string passed; constants that only exist in dev/test environments.

Understand the failure class

Background: "missing required config value" errors: why libraries refuse to start when a configuration key is empty, unset, or blank — this error's family across 48 libraries.

Related errors

AI-assisted analysis of twigphp/Twig@a414c3a491 (2026-09-13). Data as JSON: /api/errors/985161ee3792276d. Report an issue: GitHub.

Appendix: source

Thrown at src/Extension/CoreExtension.php:1695

    {
        if (null !== $object) {
            if ('class' === $constant) {
                return $checkDefined ? true : $object::class;
            }

            $constant = $object::class.'::'.$constant;
        }

        if (!\defined($constant)) {
            if ($checkDefined) {
                return false;
            }

            if ('::class' === strtolower(substr($constant, -7))) {
                throw new RuntimeError(\sprintf('You cannot use the Twig function "constant" to access "%s". You could provide an object and call constant("class", $object) or use the class name directly as a string.', $constant));
            }

            throw new RuntimeError(\sprintf('Constant "%s" is undefined.', $constant));
        }

        return $checkDefined ? true : \constant($constant);
    }

    /**
     * Batches item.
     *
     * @param array $items An array of items
     * @param int   $size  The size of the batch
     * @param mixed $fill  A value used to fill missing items
     *
     * @internal
     */
    public static function batch($items, $size, $fill = null, $preserveKeys = true): array
    {
        if (!is_iterable($items)) {
            throw new RuntimeError(\sprintf('The "batch" filter expects a sequence or a mapping, got "%s".', get_debug_type($items)));

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