I’m a Right Brainer

To open a connection to MySQL database using PHP, use the mysql_connect() function.

<?php
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';

$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');

$dbname = 'petstore';
mysql_select_db($dbname);
?>

It’s a good practice to separate the routine of opening database, closing database, etc and include them later.

index.php

<?php
include 'config.php';
include 'opendb.php';

// ... do something like insert or select, etc

include 'closedb.php';
?>

config.phps

<?php
// This is an example of config.php
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';
$dbname = 'phpcake';
?>

opendb.phps

<?php
// This is an example opendb.php
$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
mysql_select_db($dbname);
?>

closedb.phps

<?php
// an example of closedb.php
// it does nothing but closing
// a mysql database connection

mysql_close($conn);
?>

Source: php-mysql-tutorial.com