[php-src] Issue #21972: Corrupted variable type `Argument #1 ($string) must be of type string, string given`

Issue: https://github.com/php/php-src/issues/21972
Author: susnux

### Description

The following code:

```php
<?php
declare(strict_types=1);
// ...
class MyClass {
    // ...
    public function getValueBool(): bool {
        $b = strtolower($this->getTypedValue(), ValueType::BOOL));
        // ...
    }
    protected function getTypedValue(): string {
        // ...
    }
}
```

Resulted in this output:
```
TypeError: strtolower(): Argument #1 ($string) must be of type string, string given
```

But I expected this output instead:
```
$b = "false"
```

---

What is confusing here is that we use strict types thus the return type of `getTypedValue` always has to be a string. This does not throw a TypeError so it should be a string.
If we add more debugging to the return value before putting it to `strtolower`, we can see:
value: `"false"`
type: `"unknown type"` (when using `gettype($returnValue)`

In our real code base its this line:
https://github.com/nextcloud/server/blob/57a2630be741ce039929830294b93982587e9ec3/lib/private/Config/UserConfig.php#L708

We [received a bug ourself](https://github.com/nextcloud/server/issues/59629#issuecomment-4376099184) but I suspect this is a php bug from the TypeError received and the "unknown type" type

### PHP Version

```plain
8.4.20
```

### Operating System

Ubuntu / docker