How to use html inside a functions.php code?

function misha_loadmore_ajax_handler() {
    $args                = json_decode ( stripslashes ( $_POST[ 'query' ] ), true );
    $args[ 'paged' ]       = $_POST[ 'page' ] + 1;
    $args[ 'post_status' ] = 'publish';

    query_posts ( $args );
    if( have_posts () ) :while ( have_posts () ): the_post ();
            ?>
            <div class="griditem">
                <a class="producturl" href="<?php the_permalink (); ?>">
                    <div class="productimagewrapper">
                        <img class="lazyload" data-src="<?php the_post_thumbnail_url (); ?>" />
                    </div>
                    <div class="productinfo">
                        <h2 class="productname"><?php the_title (); ?></h2>
                        <span class="productprice">$<?php echo esc_html ( get_post_meta ( get_the_ID (), 'price', true ) ); ?>.00</span>
                    </div>
                </a>
            </div>
            <?php
        endwhile;
    endif;
    die;
}

or you can use ob_start ()

function misha_loadmore_ajax_handler() {
    $args                  = json_decode ( stripslashes ( $_POST[ 'query' ] ), true );
    $args[ 'paged' ]       = $_POST[ 'page' ] + 1;
    $args[ 'post_status' ] = 'publish';
    $new_html = array();
    query_posts ( $args );
    if( have_posts () ) :while ( have_posts () ): the_post ();
            ob_start ();
            ?>
            <div class="griditem">
                <a class="producturl" href="<?php the_permalink (); ?>">
                    <div class="productimagewrapper">
                        <img class="lazyload" data-src="<?php the_post_thumbnail_url (); ?>" />
                    </div>
                    <div class="productinfo">
                        <h2 class="productname"><?php the_title (); ?></h2>
                        <span class="productprice">$<?php echo esc_html ( get_post_meta ( get_the_ID (), 'price', true ) ); ?>.00</span>
                    </div>
                </a>
            </div>
            <?php
            echo ob_get_clean ();
        endwhile;
    endif;
    die;    
}