PHP mysqli_select_db() Function

<?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();
  }

// ...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);
?>