今天在看《高性能php应用开发》这本书，说道如何启用mod_deflate：

启用如下模块：

LoadModule deflate_module modules/mod_deflate.so

然后加入指令：

AddOutputFilterByType DEFLATE text/html text/plain text/xml

即可。

但是发现老是出错，

 Invalid command 'AddOutputFilterByType', perhaps misspelled or defined by a module not included in the server configuration 

去百度查看下，未果，之后发现官方有段话：

Invalid command 'AddOutputFilterByType', perhaps misspelled       or defined by a module not included in the server configuration        -AddOutputFilterByType       has moved from the core to mod_filter, which must be loaded.

原来，除了启用deflate之外，还要启用mod_filter，找到这句：

LoadModule filter_module modules/mod_filter.so

去除前面的分号，终于好了！