Check if a class constant exists

Question

How can I check if a constant is defined in a PHP class?

class Foo {
    const BAR = 1;
}

Is there something like property_exists()

Answer 1

You have 3 ways to do it:

defined()

[PHP >= 4 - most retro-compatible way]

$class_name = get_class($object); // remember to provide a fully-qualified class name
$constant = "$class_name::CONSTANT_NAME";
$constant_value = defined($constant) ? $constant : null;

ReflectionClass

[PHP >= 5]

$class_reflex = new \ReflectionClass($object);
$class_constants = $class_reflex->getConstants();
if (array_key_exists('CONSTANT_NAME', $class_constants)) {
    $constant_value = $class_constants['CONSTANT_NAME'];
} else {
    $constant_value = null;
}

ReflectionClassConstant

[PHP >= 7.1.0]

$class_name = get_class($object); // fully-qualified class name
try {
    $constant_reflex = new \ReflectionClassConstant($class_name, 'CONSTANT_NAME');
    $constant_value = $constant_reflex->getValue();
} catch (\ReflectionException $e) {
    $constant_value = null;
}

There is no real better way. Depends on your needs and use case.