mysqli database connection php

<?php
$mysqli = new mysqli("localhost","my_user","my_password","my_db");

// Check connection
if ($mysqli -> connect_errno) {

  echo "Failed to connect to MySQL: " . $mysqli -> connect_error;
  
  exit();

  }
?>
 

<?php

$con = mysqli_connect("localhost","my_user","my_password","my_db");

 
// Check connection

 if (mysqli_connect_errno()) {

  echo "Failed to connect to MySQL: " . mysqli_connect_error();

    
  exit();
}

?>
 

<?php
$servername = "localhost";
$username = "username";
$password = "password";

// Create connection
$conn = new mysqli($servername, $username, $password);

 
// Check connection

 if ($conn->connect_error) {

  die("Connection failed: " . $conn->connect_error);
}

echo "Connected successfully";

?>
 

mysqli_connect ( "host", "username", "password", "database_name" )