Add docs / guide

Author: joehoyle

guide/src/SUMMARY.md

@@ -32,3 +32,4 @@
   - [Constants](./macros/constant.md)
   - [`ZvalConvert`](./macros/zval_convert.md)
 - [Exceptions](./exceptions.md)
+- [INI Settings](./ini-settings.md)

guide/src/ini-settings.md

@@ -0,0 +1,48 @@
+# INI Settings
+
+Your PHP Extension may want to provide it's own PHP INI settings to configure behaviour. This can be done in the `#[php_startup]` annotated startup function.
+
+## Registering INI Settings
+
+All PHP INI definitions must be registered with PHP to get / set their values via the `php.ini` file or `ini_get() / ini_set()`.
+
+
+```rust,no_run
+# #![cfg_attr(windows, feature(abi_vectorcall))]
+# extern crate ext_php_rs;
+# use ext_php_rs::prelude::*;
+# use ext_php_rs::zend::IniEntryDef;
+# use ext_php_rs::flags::IniEntryPermission;
+
+#[php_startup]
+pub fn startup_function(ty: i32, module_number: i32) {
+    let ini_entries: Vec<IniEntryDef> = vec![
+        IniEntryDef::new(
+            "my_extension.display_emoji".to_owned(),
+            "yes".to_owned(),
+            IniEntryPermission::All,
+        ),
+    ];
+    IniEntryDef::register(ini_entries, module_number);
+}
+# fn main() {}
+```
+
+## Getting INI Settings
+
+The INI values are stored as part of the `GlobalExecutor`, and can be accessed via the `ini_values()` function. To retrieve the value for a registered INI setting
+
+```rust,no_run
+# #![cfg_attr(windows, feature(abi_vectorcall))]
+# extern crate ext_php_rs;
+# use ext_php_rs::prelude::*;
+# use ext_php_rs::zend::ExecutorGlobals;
+
+#[php_startup]
+pub fn startup_function(ty: i32, module_number: i32) {
+    // Get all INI values
+    let ini_values = ExecutorGlobals::get().ini_values(); // HashMap<String, Option<String>>
+    let my_ini_value = ini_values.get("my_extension.display_emoji"); // Option<Option<String>>
+}
+# fn main() {}
+```