Why can't I connect to a local MySQL server from a PHP application, but can connect from mysql-client

Hello -

I am very confused.

I have a single Ubuntu 18.04 LAMP server. The database is localhost, i.e. running on the same server. It’s replacing a current production Ubuntu 14 web server.

L = Ubuntu 18.04 A = Apache 2.4.43 M = MySQL 5.7.29 P = PhP 5.6.40

I can connect to the mysql-client with the credentials I established, and the schema I imported.

mysql -uroot -hlocalhost -p schema_name

However, when I install my application (the same as the one installed on the current 14 server) my error.log reports:

Access denied for user 'root'@'localhost' 

I’ve checked and double-checked that my application is receiving the same input for password, etc. that I’m passing to the cli client.

I created a new PHP file from this Q&A here: https://www.digitalocean.com/community/questions/why-can-t-i-connect-to-a-remote-mysql-server-from-a-php-application-but-can-connect-from-mysql-client

My file can be found here: https://gist.github.com/tneigerux/499931aae84441d1ee8c667cedac8c70

Results:

Ubuntu 14:

There are 60 tables

Ubuntu 18

Unable to Connect to 'localhost'

I don’t think the issue has to do with Ubuntu 14 vs 18, but those are where the working and non-working examples exist.

Unfortunately this is still a PHP5.6 application, so it was harder to install the necessary LAMP version to Ubuntu 18. I did log what packages I installed. I wonder if the issue is missing software? Doesn’t make sense but I can’t think of anything else.

apt-get log:

sudo add-apt-repository ppa:ondrej/php
sudo add-apt-repository ppa:ondrej/apache2
sudo apt-get -y update
sudo apt-get -y install apache2
sudo apt-get -y install git
sudo apt-get -y install php5.6
sudo apt-get -y install php5.6-mbstring
sudo apt-get -y install libaio1
sudo apt-get -y install php5.6-mysql
sudo apachectl graceful

mysql-server 5.7.29 was also installed, but to get a compatible version, via Debian packages found here: https://downloads.mysql.com/archives/community/

I’ve deployed this application to multiple servers and never had this issue. Thanks in advance for any ideas.