Take a look at this code, please:
$array = array(
\'action\' => function () { echo \"this works\"; }
);
class Test {
public $array = array(
From the class it's a property !
Rule from properties :
Declaration may include an initialization, but this initialization must be a constant value--that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.
http://php.net/manual/en/language.oop5.properties.php