Add rationale for no solution in PHP

PHP cannot have a solution given the rules due to its lack of support for nested function dereferencing. The following will always produce a syntax error:

```php
<?Php

g()()('al');
g()('al');
```

This may be possible in the future should the [Uniform Variable Syntax](https://wiki.php.net/rfc/uniform_variable_syntax) RFC get accepted.