[php-src] Issue #21521: Namespace function to make variables inside a namespace defenition.

Issue: https://github.com/php/php-src/issues/21521
Author: remco-pc

### Description

Well i have a framework which can handle multiple domains/subdomains inside a directory like /mnt/Domain/<subdomain>.<domain>.<extension>/Controller/Controller.php and inside that Controller there is a namespace like 
namespace Domain/Www_example_com/Controller 

Which can be handled by the autoloader. 

But here it comes, my framework now has composer packages which can be installed inside a domain. 

For example:
app install raxon/issue -frontend.host=... -backend.host=api...

```php
<?php
namespace Domain\{{php.namespace.host(options('frontend.host'))}}\Application\Issue\Controller;
```

But the installation files then have no IDE support and this is currently done with my own scripting language. 

Is there any reason why a namespace cannot contain a variable part like domain?