php.packages: Make packages overridable

This will allow patching extensions without having to manually having to patch all occurrences in the dependency tree.

doc/languages-frameworks/php.section.md

@@ -135,3 +135,21 @@ Example of building `composer` with additional extensions:
   enabled ++ (with all; [ imagick redis ]))
 ).packages.composer
 ```
+
+### Overriding PHP packages {#ssec-php-user-guide-overriding-packages}
+
+`php-packages.nix` form a scope, allowing us to override the packages defined within. For example, to apply a patch to a `mysqlnd` extension, you can simply pass an overlay-style function to `php`’s `packageOverrides` argument:
+
+```nix
+php.override {
+  packageOverrides = final: prev: {
+    extensions = prev.extensions // {
+      mysqlnd = prev.extensions.mysqlnd.overrideAttrs (attrs: {
+        patches = attrs.patches or [] ++ [
+          …
+        ];
+      });
+    };
+  };
+}
+```

pkgs/development/interpreters/php/generic.nix

@@ -13,6 +13,7 @@ let
     , version
     , sha256
     , extraPatches ? []
+    , packageOverrides ? (final: prev: {})
 
     # Sapi flags
     , cgiSupport ? true
@@ -50,7 +51,7 @@ let
 
               php-packages = (callPackage ../../../top-level/php-packages.nix {
                 phpPackage = phpWithExtensions;
-              });
+              }).overrideScope' packageOverrides;
 
               allExtensionFunctions = prevExtensionFunctions ++ [ extensions ];
               enabledExtensions =