Description:
------------
The use of a constant as a value results in an error. The direct contact of the string not: 
... 
public static $staticvar = "Hello World";
....

Test script:
---------------
<?php
namespace {
    
    const HELLO_WORLD = "Hello World";

    trait TraitParent {

        public static $staticvar = HELLO_WORLD;

        public static function test() {
            return self::$staticvar;
        }

    }

    trait TraitChild {
        
        use TraitParent;
    }

    Class ClassWhitChildTrait {

        use TraitChild;
    }

    Class ClassWhitOverideChildTrait extends ClassWhitChildTrait {

        use TraitParent;
    }

    echo ClassWhitChildTrait::test() . PHP_EOL;
    echo ClassWhitOverideChildTrait::test();
}