* Add ability to show PHP warnings (etc)
I don't believe there's a way for extensions to trigger PHP notices or warnings currently. This is done using the `php_error_docref` function. I've placed a function in `ext_php_rs::php_error()` however, there might be a better place?
* Remove uneeded empty value
* Fix url
Currently it's not possible for a function to accept args by reference. This PR adds early support for doing so, by transforming any function args that are `&mut T` into args that are passed by reference. E.g.:
```
\#[php_function]
fn my_function( arg: &mut Zval ) {
arg.reference_mut().map( |zval| zval.set_bool(true) );
}
```
PHP extensions that want to provide ini settings can do so using `IniEntryDef::register()`; values can then be fetched via `GlobalExecutor::ini_values()`.
Closes https://github.com/davidcole1340/ext-php-rs/issues/208
Closes https://github.com/davidcole1340/ext-php-rs/issues/209
## Summary of the changes
### Build scripts
* the `unix_build.rs` script now honors the `PHP_CONFIG` environment variable, like `cargo php install`
* use `cargo:rerun-if-env-changed` for the `PHP`, `PHP_CONFIG` and `PATH` environment variables, to avoid needless recompilation of the whole dependency tree.
### Documentation
While trying to document the aforementioned changes, I realized that there was no chapter about installing and setting up a PHP environment to develop PHP extensions. So, I refactored the first chapters of the book into a `Getting Started` section, including instructions on how to quickly set up a PHP environment.