使用yii2命令行的时候，提示：

PHP Warning 'yii\base\ErrorException' with message 'Use of undefined constant SWOOLE_BASE - assumed 'SWOOLE_BASE' (this will throw an Error in a future version of PHP)'

表示没有安装swoole拓展，需要去官网下载一下，需要注意的是如果是php 7则不能够使用最新的版本

4.8 版本需要 php-7.2 或更高版本
5.0 版本需要 php-8.0 或更高版本

可用的一个版本建议是v4.8.13：

wget https://github.com/swoole/swoole-src/archive/refs/tags/v4.8.13.tar.gz
tar zxvf v4.8.13.tar.gz
mv v4.8.13.tar.gz swoole-src-4.8.13.tar.gz
cd swoole-src-4.8.13
/usr/local/php/bin/phpize
./configure  --enable-openssl --with-php-config=/usr/local/php/bin/php-config
make

执行编译安装：

----------------------------------------------------------------------
Libraries have been installed in:
   /root/swoole-src-4.8.13/modules

If you ever happen to want to link against installed libraries
in a given directory, LIBDIR, you must either use libtool, and
specify the full pathname of the library, or use the `-LLIBDIR'
flag during linking and do at least one of the following:
   - add LIBDIR to the `LD_LIBRARY_PATH' environment variable
     during execution
   - add LIBDIR to the `LD_RUN_PATH' environment variable
     during linking
   - use the `-Wl,--rpath -Wl,LIBDIR' linker flag
   - have your system administrator add LIBDIR to `/etc/ld.so.conf'

See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------

Build complete.
Don't forget to run 'make test'.

[root@xxxx swoole-src-4.8.13]# make install
Installing shared extensions:     /usr/local/php/lib/php/extensions/no-debug-zts-20190902/
Installing header files:          /usr/local/php/include/php/

以上表示安装成功，接下来需要配置进去php.ini中：
根据php --ini找到路径，然后在拓展中加入：

extension=swoole

wq保存即可，解决！