Add [] support in invoker.

With this PR, instead of:
```php
$this->function->file_exists[1] = false;
$this->function->file_exists[2] = false;
$this->function->file_exists[3] = true;
```
It's possible to write:
```php
$this->function->file_exists[] = false;
$this->function->file_exists[] = false;
$this->function->file_exists[] = true;
```