如何连接Mysql与php mysqli

Here Is my code:

$conn=mysqli_connect("localhost","root","");
$db=mysqli_select_db("root");`

And I got the following error:

Warning The mysqli->connect_error property only works properly as of PHP versions 5.2.9 and 5.3.0. Use the mysqli_connect_error() function if compatibility with earlier PHP versions is required.

Its Not right Method You Using The Correct Methode is

<?php 
$conn=mysqli_connect("localhost","root","","hack");
?>

You can find all the information about your problem here
http://php.net/manual/en/function.mysqli-connect.php