How to return multiples lines in a shortcode?

I found this example of a shortcode on the Internet:

function project_shortcode( $atts, $content = null ) {
    extract( shortcode_atts( array(
        'class' => '',
        'id' => '',
        ), $atts ) );

        return '<div id="' . $id . '" class="' . $class . '">' . $content . '</div>';

}
add_shortcode('button', 'project_shortcode');

This is the output I want:

<div class="container item">
    <div class="row">
        <div class="intro twelvecol">
            <div class="top-border"></div>
        </div>
        <div class="screenshot eightcol">
            <img src="https://wordpress.stackexchange.com/questions/37538/<?php bloginfo("template_url' ); ?>/images/studyatbest.png">
        </div>
        <div class="screenshot fourcol last">
            <h3>BEST LANGUAGE CENTER</h3>
            <p>BEST Language Center is an educational establishment in Taichung, Taiwan. I was asked to build and design a website they could use to offer classes, programs and display photos online.</p>
            <ul>
                <li>
                    <h4>ROLE</h4>
                    <p>Design, HTML/CSS, JavaScript</p>
                <li>
                    <h4>YEAR</h4>
                    <p>2010</p>
                </li>
                <li>
                    <h4>WEBSITE</h4>
                    <p><a href="http://studyatbest.com/">studyatbest.com</a></p>
                </li>
            </ul>
        </div>
    </div>
</div>

But I’m not sure how to do it so that I can return multiples lines in the shortcode (in order to keep the code clean). Any suggestions?

Don’t use output buffering. It is too slow and sometimes hard to debug. Use heredoc.

Example:

$output = <<<MYUNIQUENAME
    <div>$foo
      <p>Some $bar</p>
    </div>
MYUNIQUENAME;