Add GUIDE section for v3 docs

docs/en/guide/env-vars.md

@@ -1,4 +1,56 @@
 # Environment Variables
 
-<!-- TODO: Full table of all env vars supported by config/env.ini and config/env.custom.ini.
-  Migrate and update from v2 env-vars.md. -->
+All environment variables mentioned in the list on this page have default values unless otherwise noted.
+You can override the default values by setting these environment variables.
+
+## Environment variables list
+
+StaticPHP centralizes environment variables in the `config/env.ini` file.
+You can set environment variables by modifying this file.
+
+We divide the environment variables supported by StaticPHP into three types:
+
+- **Global internal environment variables**: declared after StaticPHP starts, you can use `getenv()` to get them internally in StaticPHP, and you can override them before starting StaticPHP.
+- **Fixed environment variables**: declared after StaticPHP starts, you can only use `getenv()` to get them, but you cannot override them through shell scripts.
+- **Config file environment variables**: declared before StaticPHP builds, you can set these environment variables by modifying the `config/env.ini` file or through shell scripts.
+
+You can read the comments for each parameter in [config/env.ini](https://github.com/crazywhalecc/static-php-cli/blob/v3/config/env.ini) to understand its purpose.
+
+## Custom environment variables
+
+Generally, you don't need to modify any of the following environment variables as they are already set to optimal values.
+However, if you have special needs, you can set these environment variables to meet your needs
+(for example, you need to debug PHP performance under different compilation parameters).
+
+If you want to use custom environment variables, you can use the `export` command in the terminal or set the environment variables directly before the command, for example:
+
+```shell
+# export first
+export SPC_CONCURRENCY=4
+spc build:php "mbstring,pcntl" --build-cli
+
+# or direct use
+SPC_CONCURRENCY=4 spc build:php "mbstring,pcntl" --build-cli
+```
+
+Or, if you need to modify an environment variable for a long time, you can modify the `config/env.ini` file.
+
+`config/env.ini` is divided into three sections: `[global]` is globally effective, `[windows]`, `[macos]`, `[linux]` are only effective for the corresponding operating system.
+
+For example, if you need to modify the `./configure` command for compiling PHP, you can find the `SPC_CMD_PREFIX_PHP_CONFIGURE` environment variable in the `config/env.ini` file, and then modify its value.
+
+If your build conditions are more complex and require multiple `env.ini` files to switch,
+we recommend that you use the `config/env.custom.ini` file.
+In this way, you can specify your environment variables by writing additional override items
+without modifying the default `config/env.ini` file.
+
+```ini
+; This is an example of `config/env.custom.ini` file,
+; we modify the `SPC_CONCURRENCY` and linux default CFLAGS passing to libs and PHP
+[global]
+SPC_CONCURRENCY=4
+
+[linux]
+SPC_DEFAULT_C_FLAGS="-O3"
+```
+