Testing changes on a live php site without disrupting users

example.php

<?php
    # dogs is the text every other user will see.
    # "cats" is what the developer sees...
    # This way, only the developer can see changes, and if they want to show the client, they can. 
?>

<h1>Heading!</h1>

<p>This is some text about <?php if(!isset($_GET['dev'])): ?>dogs<?php else: ?>cats<?php endif; ?>




<?php
    # useful for applying new classes too
?>

<p<?php if(isset($_GET['dev'])) echo ' class="canine"'; ?>>This is some text about dogs</p>




<?php
    # or to make it easy, set it up as a constant, for example..

    define('DEVMODE',isset($_GET['dev']));
?>

<h1><?php if(DEVMODE): ?>New<?php else: ?>Old<?php endif; ?> Heading</h1>