<?php
class ImplementationError extends Exception {
    public function __construct() {}
}

class DatabaseException extends Exception {
    public $id;

    public function __construct($id, $message) {
        $this->id = $id;
        $this->message = $message;
    }
}

/* Static connection handler, which returns a Database */
class ConnectionHandler {
    private static $databases = array();

    public static function instance($id, $dsn = null, $type = 'Database') {
        /* If $dsn is not null (we being told to connect to the database) */
        if ($dsn !== null) {
            self::$databases[$id] = new $type($dsn);
        }

        if (self::$databases[$id] instanceof Database) {
            return & self::$databases[$id];
        } else {
            throw DatabaseException($id, 'No connection to the database');
        }
    }
}

class Database {
    protected $connection;
    public function __construct($dsn) {
        throw ImplmentationError;
    }

    public function query($sql) {
        throw ImplmentationError;
    }

    public function select($what, $from, $where = null) {
        $sql = 'SELECT ' . $what . ' FROM ' . $from;

        if ($where !== null) {
            $sql .= ' WHERE ' . $where;
        }

        return $this->query($sql);
    }
}

function my_default_exception_handler($exception) {
    if ($exception instanceof DatabaseException) {
        if ($e->id === 'default') {
            // The exception was with the default database.
            echo 'Database problem: ' . $exception->getMessage();
            exit();
        } else {
            // The exception was with another database, if this was not an issue, then the
            // "my_default_exception_handler" exception handler would of never got the
            // exception. If it should be checked, it should of been caught by when it was ran.
            return;
        }
    } elseif ($exception instanceof ImplmentationError) {
        echo 'ImplmentationError on line: ' . $exception->getLine() . ' in ' . $exception->getFile() . '<br />';
        return;
    } else {
        echo 'Exception: ' . $exception->getMessage();
        exit();
    }
}
set_exception_handler('my_default_exception_handler');

class MySQLDatabase extends Database {
    public function __construct($dsn) {
        $server = explode(';', $dsn);
        $this->connection = mysql_connect($server[0], $server[1], $server[2]);
    }

    public function query($sql) {
        return mysql_query($sql, $this->connection);
    }
}


$default = Database::instance('default', 'localhost:username;password', 'MySQLDatabase');
$data = $default->query('SELECT * FROM users');

// Somewhere else:
$backup = Database::instance('backup');
/* This would throw an error, because we have not opened a connection to our backup database yet.
Better to use the following, as this checks: */
try {
    $backup = Database::instance('backup');
} catch (DatabaseException $e) {
    // If DatabaseException is throw, that means no connection to database, lets make one
    $backup = Database::instance('backup', 'settings_for_backup', 'MySQLDatabase');
}
?>