今天手贱使用 homebrew 执行了一下 brew upgrade 来更新安装过的程序。 更新完之后，执行 php -m ，发现出现报错：
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/opt/php56-mcrypt/mcrypt.so' - dlopen(/usr/local/opt/php56-mcrypt/mcrypt.so, 9): Library not loaded: /usr/local/opt/libtool/lib/libltdl.7.dylib
Referenced from: /usr/local/opt/php56-mcrypt/mcrypt.so
Reason: image not found in Unknown on line 0
Warning: PHP Startup: Unable to load dynamic library '/usr/local/opt/php56-mcrypt/mcrypt.so' - dlopen(/usr/local/opt/php56-mcrypt/mcrypt.so, 9): Library not loaded: /usr/local/opt/libtool/lib/libltdl.7.dylib
Referenced from: /usr/local/opt/php56-mcrypt/mcrypt.so
Reason: image not found in Unknown on line 0
看了一下，目录/usr/local/opt/php56-mcrypt/mcrypt.so下面是有东西的，主要的问题是: Library not loaded: /usr/local/opt/libtool/lib/libltdl.7.dylib 详细可见 http://stackoverflow.com/questions/12323252/brew-doctor-dyld-library-not-loaded-error-no-available-formula-for-zlib
解决方案
brew install libtool