Modify variable-variable syntax to be compatible with PHP7

The variable-variable syntax that is currently being used needs to be updated to be compatible with PHP7.

See related PR on github.com:
https://github.com/phpbb/phpbb/pull/3389

Modifying for example $$foo['bar'] to ${$foo['bar']} will also work in previous versions.

For a complete write-up of the changes please take a look at the RFC (merged as of December 2014):
https://wiki.php.net/rfc/uniform_variable_syntax