* 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
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.
Closes https://github.com/davidcole1340/ext-php-rs/issues/200
## Rationale
In PHP zend_strings are binary strings with no encoding information. They can contain any byte at any position.
The current implementation use `CString` to transfer zend_strings between Rust and PHP, which prevents zend_strings containing null-bytes to roundtrip through the ffi layer. Moreover, `ZendStr::new()` accepts only a `&str`, which is incorrect since a zend_string is not required to be valid UTF8.
When reading the PHP source code, it is apparent that most functions marked with `ZEND_API` that accept a `const *char` are convenience wrappers that convert the `const *char` to a zend_string and delegate to another function. For example [zend_throw_exception()](eb83e0206c/Zend/zend_exceptions.c (L823)) takes a `const *char message`, and just converts it to a zend_string before delegating to [zend_throw_exception_zstr()](eb83e0206c/Zend/zend_exceptions.c (L795)).
I kept this PR focused around `ZendStr` and it's usages in the library, but it should be seen as the first step of a more global effort to remove usages of `CString` everywhere possible.
Also, I didn't change the return type of the string related methods of `Zval` (e.g. I could have made `Zval::set_string()`
accept an `impl AsRef<[u8]>` instead of `&str` and return `()` instead of `Result<()>`). If I get feedback that it should be done in this PR, I'll do it.
## Summary of the changes:
### ZendStr
* [BC break]: `ZendStr::new()` and `ZendStr::new_interned()` now accept an `impl AsRef<[u8]>` instead of just `&str`, and are therefore infaillible (outside of the cases where we panic, e.g. when allocation fails). This is a BC break, but it's impact shouldn't be huge (users will most likely just have to remove a bunch of `?` or add a few `Ok()`).
* [BC break]: Conversely, `ZendStr::as_c_str()` now returns a `Result<&CStr>` since it can fail on strings containing null bytes.
* [BC break]: `ZensStr::as_str()` now returns a `Result<&str>` instead of an `Option<&str>` since we have to return an error in case of invalid UTF8.
* adds method `ZendStr::as_bytes()` to return the underlying byte slice.
* adds convenience methods `ZendStr::as_ptr()` and `ZendStr::as_mut_ptr()` to return raw pointers to the zend_string.
### ZendStr conversion traits
* adds `impl AsRef<[u8]> for ZendStr`
* [BC break]: replaces `impl TryFrom<String> for ZBox<ZendStr>` by `impl From<String> for ZBox<ZendStr>`.
* [BC break]: replaces `impl TryFrom<&str> for ZBox<ZendStr>` by `impl From<&str> for ZBox<ZendStr>`.
* [BC break]: replaces `impl From<&ZendStr> for &CStr` by `impl TryFrom<&ZendStr> for &CStr`.
### Error
* adds new enum member `Error::InvalidUtf8` used when converting a `ZendStr` to `String` or `&str`
