一开始以为是配置问题，其实不是，折腾了好久，貌似中文网页很少有人提到这事，更别提解决之道！
最好还是用英文关键词google之：得如下网页
https://bugs.launchpad.net/ubuntu/+source/xdebug/+bug/1096464
xdebug does not display errors by default
Bug Description
Expected behavior:
Xdebug, when installed, should display detailed error information in the browser window.
Actual behavior:
No error details are displayed. Instead, the web server returns a 500 Internal Server Error.
Details:
The default PHP5 Apache configuration located at /etc/php5/apache2/php.ini
has display_errors turned off by default. This makes sense because
exposing these errors is a security risk in a production environment.
This can easily be changed manually by the administrator. However,
Xdebug is a tool for debugging, not for production, and its installation
by the administrator implies a development/debugging environment, not production. The xdebug package should enable display_errors when it is installed.
This can be done by adding a single line to the package file installed to /etc/php5/mods-available/xdebug.ini:
display_errors = On
Upon installation, the above config file is installed into PHP's mods-available directory, and it is also symlinked to /etc/php5/conf.d/20-xdebug.ini. Thus, the configuration is loaded after php.ini and the second occurrence of display_errors overrides the first.
This simple change will allow xdebug to work as expected out-of-the-box.
最后手动到php.ini处修改display_errors = On