Description:
------------
In php8.0.0beta3, referencing an undefined constant throught the `constant` function is fatal, whereas in php7.x it is not.

Test script:
---------------
```
$ php -v
PHP 7.4.3 (cli) (built: May 26 2020 12:24:22) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.3, Copyright (c), by Zend Technologies
$ php -r "echo @constant('MYCONSTANT2');"
(no output)
```

whereas in php8

```
$ php -v
PHP 8.0.0beta3 (cli) (built: Sep  4 2020 20:24:23) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.0-dev, Copyright (c) Zend Technologies
$ php -r "echo @constant('MYCONSTANT2');"

Fatal error: Uncaught Error: Undefined constant "MYCONSTANT2" in Command line code:1
Stack trace:
#0 Command line code(1): constant('MYCONSTANT2')
#1 {main}
  thrown in Command line code on line 1
```

whereas the docs say: " [constant()] Returns the value of the constant, or NULL if the constant is not defined."