Check if a class constant exists

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;

Note: using defined() on a private constant (possible from PHP7.1) will throw error: "Cannot access private const". While using ReflectionClass or ReflectionClassConstant will work.

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.

Yes, just use the class name in front of the constant name:

defined('SomeNamespace\SomeClass::CHECKED_CONSTANT');

http://www.php.net/manual/en/function.defined.php#106287

You can check if a constant is defined with the code below:

<?php
if(defined('className::CONSTANT_NAME')){
  //defined
}else{
  //not defined
}