Environment:
    CentOS 7.4 x64
    Nignx 1.14
    PHP 7.2.6
   
configuration file Nginx.conf root / root is / web/public, phpmyadmin is placed in / web/phpmyadmin,
location /phpmyadmin/ {
    alias /web/phpmyadmin/;
    index index.php index.html index.htm;
    location ~ ^/phpmyadmin/(.+?\.php)(/.*)?$ {
        alias /web/phpmyadmin/;
        fastcgi_split_path_info ^(.+?\.php)(/.*)?$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME /web/phpmyadmin/$fastcgi_script_name;
        include fastcgi_params;
    }
    location ~ ^/phpmyadmin/(.+)\.(gif|jpg|jpeg|png|bmp|swf|flv|ico)(/.*)?$ {
        alias /web/phpmyadmin/;
        expires 6d;
    }
} the configuration code as above always prompts a 404 error. 
 Nignx error message:  FastCGI sent in stderr: "Primary script unknown" while reading response header from upstream  
if you use the root setting, there will be no problem, speechless.
location /phpmyadmin/ {
    root /web;
    index index.php index.html index.htm;
    location ~ ^/phpmyadmin/(.+?\.php)(/.*)?$ {
        root /web;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass 127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
    location ~ ^/phpmyadmin/.*\.(gif|jpg|jpeg|png|bmp|swf|flv|ico|js|css)$ {
        root /web;
        expires 7d;
    }
}