PHP code is working with MySQL but not in LAMP - php

Question

I am new to web development. I recently installed LAMP stack and was trying to connect my PHP code to MySQL which established a connection and created a database (All the code was written in a directory in opt/lampp/htdocs).
<?php
$servername = "localhost:3306"; // specifying the port number seems necessary
$username = "akshay";
$password = "xxxx";
$dbname = "sys";
try {
//valid database needs to be mentioned
$conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
// set the PDO error mode to exception
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
echo "Connected successfully<br>";
$sql = "CREATE DATABASE myDBPDO";
// use exec() because no results are returned
$conn->exec($sql);
echo "Database created successfully<br>";
} catch(PDOException $e) {
echo "Connection Error: " . "<br>" . $e->getMessage();
}
$conn = null;
?>
And it worked fine. But when I proceeded further to create a table in my database, the server gave an error:
Connection failed: SQLSTATE[HY000] [1049] Unknown database 'myDBPRO'
I later found out that MySQL was running independently and not in the lamp stack. (i.e. it always showed "ok" whenever I ran sudo /opt/lampp/lampp start whereas Apache and ProFTPD showed "already running". I was not able to open myphpadmin either).
To connect MySQL to the LAMP, I stopped MySQL:
sudo service mysql stop
And then restarted my lamp stack: sudo /opt/lampp/lampp start (This time MySQL was connected as it showed already connected when I ran the command again and I was able to open pyphpadmin too.) So, I tried to run my code again but this time, it would not even connect. It gave the following error:
Connection Error:
SQLSTATE[HY000] [1045] Access denied for user 'akshay'#'localhost' (using password: YES)
I have the following doubts:
How was my code initially working if MYSQL was not connected to LAMP stack?
Why was the created database not present in PHPMyAdmin?
How to make MySQL work with LAMP?
(Forgive me if something is too obvious, I am quite new to this.)