OOP database connect/disconnect class

Question

I have just started learning the concept of Object oriented programming and have put together a class for connecting to a database, selecting database and closing the databa

Answer 1

I would add a connection/link variable to your class, and use a destructor. That will also save you from haveing to remember to close your connection, cause it's done automatically.
It is the $this->link that you need to pass to your mysql_close().

class Database {

    private $link;
    private $host, $username, $password, $database;

    public function __construct($host, $username, $password, $database){
        $this->host        = $host;
        $this->username    = $username;
        $this->password    = $password;
        $this->database    = $database;

        $this->link = mysql_connect($this->host, $this->username, $this->password)
            OR die("There was a problem connecting to the database.");

        mysql_select_db($this->database, $this->link)
            OR die("There was a problem selecting the database.");

        return true;
    }

    public function query($query) {
        $result = mysql_query($query);
        if (!$result) die('Invalid query: ' . mysql_error());
        return $result;
    }

    public function __destruct() {
        mysql_close($this->link)
            OR die("There was a problem disconnecting from the database.");
    }

}

Example Usage:

query("SELECT * FROM students");

    while ($row = mysql_fetch_assoc($result)) {
        echo "First Name: " . $row['firstname'] ."
";
        echo "Last Name: "  . $row['lastname']  ."
";
        echo "Address: "    . $row['address']   ."
";
        echo "Age: "        . $row['age']       ."
";
        echo "
";
    }
?>

Edit:
So people can actually use the class, I added the missing properties/methods.
The next step would be to expand on the query method, to include protection against injection, and any other helper functions.
I made the following changes:

• Added the missing private properties
• Added __construct($host, $username, $password, $database)
• Merged connectdb() and select() into __construct() saving an extra two lines of code.
• Added query($query)
• Example Usage

Please if I made a typo or mistake, leave a constructive comment, so I can fix it for others.

edit 23/06/2018

As pointed out mysql is quite outdated and as this question still receives regular visits I thought I'd post an updated solution.

class Database {

    private $mysqli;
    private $host, $username, $password, $database;

    /**
     * Creates the mysql connection.
     * Kills the script on connection or database errors.
     * 
     * @param string $host
     * @param string $username
     * @param string $password
     * @param string $database
     * @return boolean
     */
    public function __construct($host, $username, $password, $database){
        $this->host        = $host;
        $this->username    = $username;
        $this->password    = $password;
        $this->database    = $database;

        $this->mysqli = new mysqli($this->host, $this->username, $this->password)
            OR die("There was a problem connecting to the database.");

        /* check connection */
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s\n", mysqli_connect_error());
            exit();
        }

        $this->mysqli->select_db($this->database);

        if (mysqli_connect_errno()) {
            printf("Connect failed: %s\n", mysqli_connect_error());
            exit();
        }

        return true;
    }

    /**
     * Prints the currently selected database.
     */
    public function print_database_name()
    {
        /* return name of current default database */
        if ($result = $this->mysqli->query("SELECT DATABASE()")) {
            $row = $result->fetch_row();
            printf("Selected database is %s.\n", $row[0]);
            $result->close();
        }
    }

    /**
     * On error returns an array with the error code.
     * On success returns an array with multiple mysql data.
     * 
     * @param string $query
     * @return array
     */
    public function query($query) {
        /* array returned, includes a success boolean */
        $return = array();

        if(!$result = $this->mysqli->query($query))
        {
            $return['success'] = false;
            $return['error'] = $this->mysqli->error;

            return $return;
        }

        $return['success'] = true;
        $return['affected_rows'] = $this->mysqli->affected_rows;
        $return['insert_id'] = $this->mysqli->insert_id;

        if(0 == $this->mysqli->insert_id)
        {
            $return['count'] = $result->num_rows;
            $return['rows'] = array();
            /* fetch associative array */
            while ($row = $result->fetch_assoc()) {
                $return['rows'][] = $row;
            }

            /* free result set */
            $result->close();
        }

        return $return;
    }

    /**
     * Automatically closes the mysql connection
     * at the end of the program.
     */
    public function __destruct() {
        $this->mysqli->close()
            OR die("There was a problem disconnecting from the database.");
    }
}

Example usage:

query("SELECT * FROM students");

    if(true == $result['success'])
    {
        echo "Number of rows: " . $result['count'] ."
";
        foreach($result['rows'] as $row)
        {
            echo "First Name: " . $row['firstname'] ."
";
            echo "Last Name: "  . $row['lastname']  ."
";
            echo "Address: "    . $row['address']   ."
";
            echo "Age: "        . $row['age']       ."
";
            echo "
";
        }
    }

    if(false == $result['success'])
    {
        echo "An error has occurred: " . $result['error'] ."
";
    }
?>