Add infos about PECL build flags

Signed-off-by: Anatoli Nicolae <[email protected]>

notes/install-php-pecl-modules.md

@@ -27,3 +27,43 @@ upcp -sb php/install-extensions
 cd /usr/local/apnscp/resources/playbooks
 ansible-playbook bootstrap.yml --tags=php/install-extensions --extra-vars=php_version=7.4 --extra-vars=multiphp_build=true
 ```
+
+## Custom build flags
+
+There may be some cases that require custom build flags to be used during build, such as compiling `redis` with `igbinary` compatibility (required i.e. when using Object Cache Pro on WordPress).
+
+To build PECL modules with custom flags, you'll have to use the dense format as scope input:
+
+```bash
+cpcmd scope:set cp.bootstrapper pecl_extensions '["imagick","igbinary","zstd",'\''{"name":"redis","flags":"--enable-redis-igbinary --enable-redis-zstd","extension":"redis"}'\'']'
+```
+
+Specifically, the chosen configuration for the `redis` module is:
+
+```json
+{
+	"name": "redis",
+	"flags": "--enable-redis-igbinary --enable-redis-zstd",
+	"extension": "redis"
+}
+```
+
+This will result in `redis` being built with `igbinary` as a serializer, and `zstd` as a compression algorithm.
+
+<Warning>
+
+Escaping quotes can become elaborate very quickly. Refer to [original docs for guidelines](https://docs.apiscp.com/admin/PHP-FPM/#dense-extension-format) and pay maximum attention.
+
+</Warning>
+
+Once the extensions to be installed are defined, proceed with building the modules or rebuilding them if previously build:
+
+```bash
+upcp -sb php/install-extensions
+upcp -sbf php/install-extensions # Force / rebuild
+```
+<Card title="Installing PHP modules" icon="book" color="#169c86" href="https://docs.apiscp.com/admin/PHP-FPM/#dense-extension-format">
+
+Learn more about dense format option.
+
+</Card>