nginx upstream prematurely closed connection

[Ramy Allam]

Hello,
I'm using nginx as a reverse proxy to handle static files with apache to handle dynamic file .php
I'm receiving too many errors ( Bad Gateway 502 ) when i'm using vBulletin , joomla , wordpress in PHP Post or curl() inside php.
I reviewed log i noticed this error is frequently appears:
"upstream prematurely closed connection while reading response header from upstream"
Nginx version: nginx/1.0.11Apache/2.2.22PHP 5.2.17
Thanks 		 	   		  
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