Fix: 502 Bad Gateway on PHP sites error and fix

The ‘502 Bad Gateway’ error occurs when a server acting as a gateway receives an invalid response from an upstream server. This commonly happens with PHP applications behind reverse proxies like Nginx.

How the Error Happens

❌ Error Scenario:

# ❌ 502 Bad Gateway response
# Server returns 502 instead of expected content

✅ Quick Fix - Resolve 502 Bad Gateway

Solution 1: Nginx Configuration

# ✅ Update nginx.conf
upstream php_backend {
    server 127.0.0.1:9000 max_fails=3 fail_timeout=30s;
}

server {
    location ~ \.php$ {
        fastcgi_pass php_backend;
        fastcgi_read_timeout 300;
        fastcgi_connect_timeout 300;
        fastcgi_send_timeout 300;
    }
}

Solution 2: PHP-FPM Configuration

# ✅ Update php-fpm pool configuration
# /etc/php/7.4/fpm/pool.d/www.conf
listen.backlog = 65535
pm.max_children = 50
pm.start_servers = 5
pm.min_spare_servers = 5
pm.max_spare_servers = 35

Solution 3: Restart Services

# ✅ Restart PHP-FPM and web server
sudo systemctl restart php7.4-fpm
sudo systemctl restart nginx