使用的php版本： php-7.3.2-nts-Win32-VC15-x64
IDE： phpstorm 2018.3.4

1

php会假设（assume）未定义的变量为字符串字面量。

栗子

<?php
/**
 * Created by PhpStorm.
 * User: zyu50
 * Date: 2019/2/17
 * Time: 17:51
 */
$v = undefined_v;
if($v == 'undefined_v'){
    echo "php还能这样，我醉了\n";
}

输出：
Warning: Use of undefined constant undefined_v - assumed 'undefined_v' (this will throw an Error in a future version of PHP) in D:\PhpstormProjects\untitled\calculate.php on line 8
php还能这样，我醉了
PHP Warning: Use of undefined constant undefined_v - assumed 'undefined_v' (this will throw an Error in a future version of PHP) in D:\PhpstormProjects\untitled\calculate.php on line 8