Resolving fastcgi_pass depending on the header value

nginx.conf

upstream dev_backend {
    server dev-php:9000;
}

upstream test_backend {
    server test-php:9000;
}

map $http_x_testing_mode $php_backend {
    "true"      "test_backend";
    default     "dev_backend";
}

server {
    listen 80;
    server_name localhost;
    root /code/public;
    index index.php;

    location / {
         try_files $uri /index.php$is_args$args;
    }

    location ~ \.php$ {
        include fastcgi_params;
        fastcgi_param SCRIPT_FILENAME $document_root/$fastcgi_script_name;
        fastcgi_pass $php_backend;
        try_files $uri=404;
    }
}