The __get() result value must still satisfy the property type if it is invoked on a visible, but unset/uninitialized property. Otherwise it would be possible to bypass the property type this way.

It should be noted that something like this will not throw an error:

<?php
class X
{
    protected string $bar;
    protected array $array = [];
    public function __set($name, $value)
    {
        print "set $name = $value".PHP_EOL; $this->array[$name] =
$value;
    }
    public function __get($name)
    {
        print 'get '.$name.PHP_EOL; return $this->array[$name] ?? null;
    }
}
$obj = new X();
$obj->bar = 'FOO'; print $obj->bar.PHP_EOL;


The reason is that here the access is performed from a scope where the property is not visible, so the type doesn't apply either. The type only applies if it is visible.

See also https://wiki.php.net/rfc/typed_properties_v2#overloaded_properties.

Ok, but why is 
   public string $bar;
or 
   protected string $bar;
not set proper in the first place, regardless of the scope?

Basically this is useless: 
   public string $bar;
because there seems no way to set the property in any way 
if an _get & _set is present.

Test this

class X
{
    protected string $bar;
    public function __construct(){
        $this->bar = 'FOO'; 
        print $this->bar.PHP_EOL;
    }
    public function __set($name, $value)
    {
        $this->{$name} = $value;
    }
}
$obj = new X();

and it works. How strange....

Ok, like the last time (last Bug Report). 
Maybe i never will understand this hole thing. 
So i try to work the save road on this topic, to prevent behavior that i don't fully understand. 
I understand now why "X::$bar must be string, null used" happened, and that is one step in the right direction for me. 

So thanks for the fast reply. 

p.s. ignore the 'Basically this is useless:' part from my last post

Yes, the main problem here is still bug #78226, which is yet unsolved.

You *can* set the property inside the __set() handler. Basically something like

public function __set($name, $value) {
    if (property_exists($this, $name)) {
        $this->$name = $value;
        return;
    }
    // normal __set implementation
}

may work.