在双引号中使用复杂(curly)语法解析常量

Question

我无法理解在双引号中使用复杂(卷曲)语法的概念-

class beers {
    const softdrink = 'rootbeer';
    public static $ale = 'ipa';
}

define('TEST_DATA','test');
$test='hello';

$rootbeer = 'A & W';
$ipa = 'Alexander Keith\'s';

//This works; outputs: I'd like an A & W
echo "I'd like an ${beers::softdrink}\n";

//This works too; outputs: I'd like an Alexander Keith's
echo "I'd like an ${beers::$ale}\n";

//but this not work
echo "I'd like an ${TEST_DATA}\n";

${TEST_DATA}在常量的情况下不工作，但在类常量${beers::softdrink}的情况下，它可以工作。