The plugin generated 2 characters of unexpected output during activation.

自己开发的插件，激活时出现以下错误：

The plugin generated 2 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

后来发现，在两个php文件的开头各有一个空格，删掉之后，就没事了。

参考了这个帖子：php – The plugin generated X characters of unexpected output during activation (WordPress) – Stack Overflow