[solved]

That error message means that the PHP-FPM process is not running. What happens when you type

/etc/init.d/php-fpm start

the php page is still not running, it was redirected to nginx's 50x.html

Problem solved! Here's how:

go to /usr/local/etc/php-fpm.conf, find this line

listen = /usr/local/var/run/php-fpm.sock

comment it, and add this line after it

listen = 127.0.0.1:9000

However, I'm still not sure why this line is the cause of all the problems. Does anyone know why?

Yep, it's because listen = /usr/local/var/run/php-fpm.sock is a unix socket and you had nginx trying to pass via tcp fastcgi_pass 127.0.0.1:9000;

If you want to use the unix socket (I do they tend to be a little bit faster) set fastcgi_pass to unix:/usr/local/var/run/php-fpm.sock

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[solved]