<?php
$con=mysqli_connect("localhost","my_user","my_password","my_db");
// Check connection
if (mysqli_connect_errno($con))
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

// ...some PHP code for database "my_db"...

// Change database to "test"
mysqli_select_db($con,"test");

// ...some PHP code for database "test"...

mysqli_close($con);
?>