Can You Select Someone Else Database? - MySQL FAQs - PHP Connections and Query Execution

If your MySQL server is provided by an Internet service company, they will provide you one database for your use only. There are many other databases on the server for other users. But your user account will have no privilege to select other databases. If you try to access other databases, you will get an error. The following tutorial exercise shows you a guest user trying to access the system "mysql" database:

<?php
  $con = mysql_connect('localhost:8888', 'guest', 'pub');
  if (mysql_select_db('mysql', $con)) {
    print("Database mysql selected.\n");
  } else {
    print("Database selection failed with error:\n");
    print(mysql_errno($con).": ".mysql_error($con)."\n");
  }
  mysql_close($con); 
?>

You will get something like this:

Database selection failed with error:
1044: Access denied for user 'guest'@'%' to database 'mysql'