如何通过字符串获取defined（）值？

Question

define('key_start_atz', 200);

$str = 'key_start_atz';

如何通过200获取$str？

Answer 1

constant($str);

Answer 2

define('key_start_atz', 200);

$x = 'key_start_atz'; // contains 'key_start_atz'
$x = key_start_atz; // contains 200
$x = constant('key_start_atz'); // contains 200