php - 为什么存在empty() "not"？

Question

谁能给我解释一下吗？

var_dump([
    defined('empty'),
    function_exists('empty'),
    is_callable('empty'),
    empty(0),
    empty(1),
    ]);

array (size=5)
  0 => boolean false
  1 => boolean false
  2 => boolean false
  3 => boolean true
  4 => boolean false

我明明可以使用empty，那为什么PHP会报告它未定义、不存在、不可调用呢？ 😟

Answer 1

然后我当然找到了......

A PHP function is passed by its name as a string. Any built-in or user-defined function can be used, except language constructs such as: array(), echo, empty(), eval(), exit(), isset(), list(), print or unset().

-- Callbacks / Callables: Passing