Answers for "which of the following is the correct way of writing a php database connection string?"

php sql connection string

$servername = "localhost";
$username = "username";
$password = "password";

try {
  $conn = new PDO("mysql:host=$servername;dbname=myDB", $username, $password);
  // set the PDO error mode to exception
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  echo "Connected successfully";
} catch(PDOException $e) {
  echo "Connection failed: " . $e->getMessage();
}

database connection in php

<?php

$hostName = 'localhost';
$userNmame = 'root';
$password = '';
$dbname = 'topproduct';
$db_name = "mysql:host=$hostName;dbname=$dbname";
$conn = new PDO($db_name,$userNmame,$password);

if(!$conn){
    echo 'Error database connection';    
}