修改方式要注意：
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https://stackoverflow.com/questions/4731364/internal-error-500-apache-but-nothing-in-the-logs
Locate your php.ini file:

el@apollo:~$ locate php.ini
/etc/php5/apache2/php.ini
Edit that file as root:

sudo vi /etc/php5/apache2/php.ini
Find this line in php.ini:

display_errors = Off
Change the above line to this:

display_errors = On
Lower down in the file you'll see this:

;display_startup_errors
; Default Value: Off
; Development Value: On
; Production Value: Off

;error_reporting
; Default Value: E_ALL & ~E_NOTICE
; Development Value: E_ALL | E_STRICT
; Production Value: E_ALL & ~E_DEPRECATED
The semicolons are comments, that means the lines don't take effect. Change those lines so they look like this:

display_startup_errors = On
; Default Value: Off
; Development Value: On
; Production Value: Off

error_reporting = E_ALL
; Default Value: E_ALL & ~E_NOTICE
; Development Value: E_ALL | E_STRICT
; Production Value: E_ALL & ~E_DEPRECATED
What this communicates to PHP is that we want to log all these errors. Warning, there will be a large performance hit, so you don't want this enabled on production because logging takes work and work takes time, time costs money.

Restarting PHP and Apache should apply the change.

Do what you did to cause the 500 Internal Server error again, and check the log:

vi /var/log/apache2/error.log
You should see the 500 error at the end, something like this:

[Wed Dec 11 01:00:40 2013] [error] [client 192.168.11.11] PHP Fatal error:
Call to undefined function Foobar\byob\penguin\alert() in /yourproject/
your_src/symfony/Controller/MessedUpController.php on line 249, referer:
https://nuclearreactor.com/abouttoblowup