Example for linking a custom extension to PHP (#919)

## Summary

Shows how to use flakes to add a custom extension (.so file) to PHP. I
think this is dynamically linking the extension instead of compiling it
into PHP -- not sure how to compile it statically

## How was it tested?
Follow the instructions in the Readme

Author: Lagoja

examples/flakes/php-extension/README.md

@@ -0,0 +1,9 @@
+# Adding a custom PHP Extension
+
+This example shows how to add a custom PHP extension to a PHP package using Flakes. This uses an extension built from the [PHP Skeleton Extension](https://github.com/improved-php-library/skeleton-php-ext) from [Improved PHP Library](https://github.com/improved-php-library)
+
+To test this example:
+
+1. Run `devbox shell` to start your shell
+2. Start PHP in interactive mode with `php -a`
+3. Run `echo skeleton_nop("Hello World");` to test the extension. This should print `Hello World` to the screen

examples/flakes/php-extension/devbox.json

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+{
+    "packages": [
+        "path:my-php-flake#php"
+    ],
+    "shell": {
+        "init_hook": null
+    },
+    "nixpkgs": {
+        "commit": "f80ac848e3d6f0c12c52758c0f25c10c97ca3b62"
+    }
+}

examples/flakes/php-extension/my-php-flake/flake.lock

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+{
+  description =
+    "A flake that outputs PHP with a custom extension (skeleton.so) linked.";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        packages = {
+          # Customize and export the PHP package with some extra config
+          php = pkgs.php.buildEnv {
+            # extraConfig will add the line below to our php.ini
+            # ${self} is a variable representing the current flake
+            extraConfig = ''
+              extension=${self}/skeleton.so 
+            '';
+          };
+        };
+      });
+}