Fixing cacti

If you use ubuntu 18.04 and use nginx as your webserver and want to install cacti there are a few things to watch out for.
cacti is php based but when you install php via apt it will install apache2 as well.
If you remove the apache2 package then there will be some un-needed dependencies. If you autoremove those dependencies then your cacti will not work.

How to fix

cacti homepage error

FATAL: Connection to Cacti database failed. Please ensure:

the PHP MySQL module is installed and enabled.
the database is running.
the credentials in config.php are valid.

testing

mysql -u cactiuser -pcactipasswd -h localhost -P 3306 -D cactidb

if mysql works then test using php

php -r "new PDO('mysql:host=localhost;port=3306;dbname=cactidb', 'cactiuser', 'cactipasswd');"
PHP Fatal error:  Uncaught PDOException: could not find driver in Command line code:1
Stack trace:
#0 Command line code(1): PDO->__construct('mysql:host=loca...', 'cactiuser', 'cactipasswd')
#1 {main}
  thrown in Command line code on line 1

sudo vi /etc/php/7.2/cli/php.ini
uncomment extension=php_pdo_mysql.dll

$ php -r "new PDO('mysql:host=localhost;port=3306;dbname=cactidb', 'cactiuser', 'cactipasswd');"
PHP Warning:  PHP Startup: Unable to load dynamic library 'pdo_mysql' (tried: /usr/lib/php/20170718/pdo_mysql (/usr/lib/php/20170718/pdo_mysql: cannot open shared object file: No such file or directory), /usr/lib/php/20170718/pdo_mysql.so (/usr/lib/php/20170718/pdo_mysql.so: cannot open shared object file: No such file or directory)) in Unknown on line 0

sudo apt install php-mysql

PHP Warning:  PHP Startup: Unable to load dynamic library 'pdo_mysql' (tried: /usr/lib/php/20170718/pdo_mysql (/usr/lib/php/20170718/pdo_mysql: cannot open shared object file: No such file or directory), /usr/lib/php/20170718/pdo_mysql.so (/usr/lib/php/20170718/pdo_mysql.so: undefined symbol: mysqlnd_allocator)) in Unknown on line 0

sudo apt install php-mysqlnd
sudo apt install php
sudo apt remove –purge php7.2-mysql
sudo apt remove –purge php-mysqlnd
sudo apt remove –purge php7.2-cli
sudo apt remove –purge php7.2
sudo apt install php-mysql
sudo apt install php-cli
sudo apt install php
stop/disable apache2
use systemctl commands to prevent a service from automatically starting at boot.

sudo systemctl disable apache2

502 bad gateway

2019/11/29 15:00:13 [crit] 8894#8894: *697 connect() to unix:/var/run/php/php7.2-fpm.sock failed (2: No such file or directory) whileconnecting to upstream, client: 192.168.12.28, server: _, request: "GET /cacti/index.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.2-fpm.sock:", host: "192.168.1.2"

sudo apt remove –purge php7.2-fpm
sudo apt install php-fpm

2019/11/29 15:04:01 [error] 8894#8894: *766 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught Error: Call to undefinedfunction mb_strlen() in /usr/share/cacti/lib/rrd.php:1593
Stack trace:
#0 /usr/share/cacti/graph_json.php(158): rrdtool_function_graph(12, '0', Array)

mbstring not enabled by default in php
reinstall mbstring
sudo apt install php-mbstring
reinstall rrdtool
sudo apt install rrdtool
snmp tools missing
sudo apt install snmp