Create a simple shortcode in WordPress

simple-shortcode.php

<?php
/**
 * Prefix Shortcode
 *
 * @todo Change the `Prefix` with your own unique prefix.
 * @todo Change the `prefix` with your own unique prefix.
 * 
 * @package Prefix
 * @since 1.0.0
 */

if ( ! class_exists( 'Prefix_Shortcode' ) ) :

	/**
	 * Prefix_Shortcode
	 *
	 * @since 1.0.0
	 */
	class Prefix_Shortcode {

		/**
		 * Instance
		 *
		 * @access private
		 * @var object Class Instance.
		 * @since 1.0.0
		 */
		private static $instance;

		/**
		 * Initiator
		 *
		 * @since 1.0.0
		 * @return object initialized object of class.
		 */
		public static function get_instance() {
			if ( ! isset( self::$instance ) ) {
				self::$instance = new self;
			}
			return self::$instance;
		}

		/**
		 * Constructor
		 *
		 * @since 1.0.0
		 */
		public function __construct()
		{
			// Register shortcode.
			add_shortcode( 'prefix-portfolio', array( $this, 'shortcode_markup' ) );
		}

		/**
		 * Shortcode
		 *
		 * @since 1.0.0
		 * @param  array $data Shortcode attributes.
		 * @return mixed    Shortcode markup.
		 */
		function shortcode_markup( $data = array() )
		{
			$data = shortcode_atts(
				array(
					'attribute-1' => '',
					'attribute-2' => '',
				), $data
			);

			ob_start();
			?>
			<div class="prefix-shortcode">
				<?php echo esc_html( $data['attribute-1'] ); ?>
				<?php echo esc_html( $data['attribute-2'] ); ?>
			</div>
			<?php

			return ob_get_clean();
		}
	}

	/**
	 * Kicking this off by calling 'get_instance()' method
	 */
	Prefix_Shortcode::get_instance();

endif;