502错误基本就是php进程执行中挂了，其中有个原因就可能是进程执行超时设置导致的比如这个：
; The timeout for serving a single request after which the worker process will
; be killed. This option should be used when the ‘max_execution_time‘ ini option
; does not stop script execution for some reason. A value of ‘0‘ means ‘off‘.
; Available units: s(econds)(default), m(inutes), h(ours), or d(ays)
; Default Value: 0
;request_terminate_timeout = 0
原文：http://www.cnblogs.com/anjianliang/p/5841703.html