php: Add enabledExtensions attribute to PHP derivations

This provides a means to build a PHP package based on a list of
extensions from another.

For example, to generate a package with all default extensions
enabled, except opcache, but with ImageMagick:

php.withExtensions (e:
  (lib.filter (e: e != php.extensions.opcache) php.enabledExtensions)
  ++ [ e.imagick ])

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/languages-frameworks/php.section.md b/doc/languages-frameworks/php.section.md
index cc98c760aec..a302a9a7f87 100644
--- a/doc/languages-frameworks/php.section.md
+++ b/doc/languages-frameworks/php.section.md
@@ -48,6 +48,21 @@ enabled:
 php.withExtensions (e: with e; [ imagick opcache ])
 ```
 
+Note that this will give you a package with _only_ opcache and
+ImageMagick, none of the other extensions which are enabled by default
+in the `php` package will be available.
+
+To enable building on a previous PHP package, the currently enabled
+extensions are made available in its `enabledExtensions`
+attribute. For example, to generate a package with all default
+extensions enabled, except opcache, but with ImageMagick:
+
+```nix
+php.withExtensions (e:
+  (lib.filter (e: e != php.extensions.opcache) php.enabledExtensions)
+  ++ [ e.imagick ])
+```
+
 If you want a PHP build with extra configuration in the `php.ini`
 file, you can use `php.buildEnv`. This function takes two named and
 optional parameters: `extensions` and `extraConfig`. `extensions`