php-fpm 正常启动，nginx也正常启动，但是为什么访问PHP是502

php-fpm是正常启动的，如下

liweideiMac:~ liwei$ ps -ef | grep php-fpm
  501  2756     1   0 12:38下午 ??         0:00.00 /usr/local/Cellar/php55/5.5.14/sbin/php-fpm
  501  2757  2756   0 12:38下午 ??         0:00.00 /usr/local/Cellar/php55/5.5.14/sbin/php-fpm
  501  2758  2756   0 12:38下午 ??         0:00.00 /usr/local/Cellar/php55/5.5.14/sbin/php-fpm
  501  2759  2756   0 12:38下午 ??         0:00.00 /usr/local/Cellar/php55/5.5.14/sbin/php-fpm
  501  2761  1217   0 12:38下午 ttys000    0:00.00 grep php-fpm

nginx也是正常启动的

$ ps -ef | grep nginx
    0   231   158   0 10:19上午 ??         0:00.02 nginx: master process /usr/local/opt/nginx/bin/nginx -g daemon off;  
   70  1858   231   0 11:39上午 ??         0:00.01 nginx: worker process  
   70  1859   231   0 11:39上午 ??         0:00.54 nginx: worker process  
   70  1860   231   0 11:39上午 ??         0:00.53 nginx: worker process  
   70  1861   231   0 11:39上午 ??         0:00.54 nginx: worker process  
   70  1862   231   0 11:39上午 ??         0:00.05 nginx: cache manager process  
  501  2772  1217   0 12:40下午 ttys000    0:00.00 grep nginx

访问PHP的时候提示

An error occurred.

Sorry, the page you are looking for is currently unavailable. Please
try again later.

If you are the system administrator of this resource then you should
check the error log for details.

Faithfully yours, nginx.

打开日志看到如下：

127.0.0.1 - - [30/Jun/2014:12:38:19 +0800] "GET /info.php HTTP/1.1" 502 537 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.114 Safari/537.36"

请问如何解决？

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搞清楚问题了，其实都没问题，只是nginx我开启了缓存，我第一次启动失败后，他缓存了502
下次我再访问他就一直都是502了

我将地址后面加了一个?a=1，发现一切正常