Hello Dolly OOP example (untested)

hello.class.php

<?php
/*
Plugin Name: Hello Dolly OOP example
Plugin URI: https://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong: Hello, Dolly. When activated you will randomly see a lyric from <cite>Hello, Dolly</cite> in the upper right of your admin screen on every page.
Author: Matt Mullenweg (and set to OOP by Sybre Waaijer)
Version: 1.6.oop
Author URI: http://ma.tt/
*/

new Hello_Dolly();

class Hello_Dolly {

	//* Store current lyric here.
	public $current_lyric;

	public function __construct() {

		add_action( 'admin_head', array( $this, 'dolly_css' ) );
		add_action( 'admin_notices', array( $this, 'hello_dolly' ) );

	}

	//* Needs to be public as it's attached to an external action.
	public function dolly_css() {
		$x = is_rtl() ? 'left' : 'right';

		echo "
		<style type='text/css'>
		#dolly {
			float: $x;
			padding-$x: 15px;
			padding-top: 5px;
			margin: 0;
			font-size: 11px;
		}
		</style>
		";
	}

	//* Can be protected as it's called directly from within the class
	protected function hello_dolly_set_lyric() {
		/** These are the lyrics to Hello Dolly */
		$lyrics = "Hello, Dolly\r\n"
				. "Well, hello, Dolly\r\n"
				. "It's so nice to have you back where you belong\r\n"
				. "You're lookin' swell, Dolly\r\n"
				. "I can tell, Dolly\r\n"
				. "You're still glowin', you're still crowin'\r\n"
				. "You're still goin' strong\r\n"
				. "We feel the room swayin'\r\n"
				. "While the band's playin'\r\n"
				. "One of your old favourite songs from way back when\r\n"
				. "So, take her wrap, fellas\r\n"
				. "Find her an empty lap, fellas\r\n"
				. "Dolly'll never go away again\r\n"
				. "Hello, Dolly\r\n"
				. "Well, hello, Dolly\r\n"
				. "It's so nice to have you back where you belong\r\n"
				. "You're lookin' swell, Dolly\r\n"
				. "I can tell, Dolly\r\n"
				. "You're still glowin', you're still crowin'\r\n"
				. "You're still goin' strong\r\n"
				. "We feel the room swayin'\r\n"
				. "While the band's playin'\r\n"
				. "One of your old favourite songs from way back when\r\n"
				. "Golly, gee, fellas\r\n"
				. "Find her a vacant knee, fellas\r\n"
				. "Dolly'll never go away\r\n"
				. "Dolly'll never go away\r\n"
				. "Dolly'll never go away again"
				;

		// Here we split it into lines
		$lyrics = explode( "\n", $lyrics );

		// And then randomly choose a line
		return $this->current_lyric = wptexturize( $lyrics[ mt_rand( 0, count( $lyrics ) - 1 ) ] );
	}

	// This just echoes the chosen line, we'll position it later
	public function hello_dolly() {
		//* Set lyric in $this->current_lyric
		$this->hello_dolly_set_lyric();

		echo '<p id='dolly'>' . $this->current_lyric . '</p>';
	}
}