How did you determine that the value is still "Hello"? Please provide a complete example which also shows the incorrect result.

public function func()
	{
		$this->var = null;
		print_r($this->var);// Does nothing, still "Hello"
		unset($this->var);
		print_r($this->var);// Does nothing, still "Hello"
		$this->var = false;
		print_r($this->var);// Works and prints "Hello"
	}

is it *really* that hard to provide a *full working* reproducer instead some useless comments?

your comments are simply not true

[user@localhost:/downloads]$ cat test.php
<?php
$test = new myClass;
$test->func();
class myClass {
        private $var = null;

        function __construct() {
                $this->var = "Hello";
        }

        public function func()
        {
                $this->var = null;  // Does nothing, still "Hello"
                echo $this->var, "\n";
                unset($this->var);  // Does nothing, still "Hello"
                echo $this->var, "\n";
                $this->var = false; // Works
        }
}
[user@localhost:/downloads]$ php test.php


Notice: Undefined property: myClass::$var in /mnt/data/downloads/test.php on line 16

[user@localhost:/downloads]$

Please ignore the troll, but running your script on an online sandbox doesn't reproduce the problem: https://3v4l.org/POdHF (I've switched to var_dump just because it shows the difference between NULL and false more clearly).

Does running that exact code on your local install gives a different result, or is there some other code that's interacting in some way?

It's in a far bigger class that I discovered this.  I'll run some tests now and get back to you.

In a simple class I can't reproduce it, but in my larger one it fails 100% of the time.

I did a workaround as setting it to false worked fine, so I left it at that.

It sounds like there's something else in that class that's interacting with your code then - perhaps some magic using Property Overloading (__set, __unset, etc): https://www.php.net/manual/en/language.oop5.overloading.php

Please come back to us if you find a way to reproduce the issue and still believe it is a bug with PHP.