Is there a way to define a constant array in PHP?

No, it’s not possible. From the manual: Constants Syntax

Only scalar data (boolean, integer, float and string) can be contained in constants. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.

If you need to set a defined set of constants, consider creating a class and filling it with class constants. A slightly modified example from the manual:

class MyClass
{
const constant1 = 'constant value';
const constant2 = 'constant value';
const constant3 = 'constant value';

  function showConstant1() {
    echo  self::constant1 . "\n";
  }
}

echo MyClass::constant3;

Also check out the link GhostDog posted, it’s a nice workaround.

define('SOMEARRAY', serialize(array(1,2,3)));

$is_in_array = in_array($x, unserialize(SOMEARRAY));

That’s the closest to an array constant.

You can not, but you can just define static array in a class and it would serve you just the same, just instead of FOO you’d write Foo::$bar.

don’t think you can. But you can always try searching.