Criando shortcode personalizado no WordPress

Função para criar um shortcode personalizado no WordPress.

/**
 * custom quote shortcode
 * via https://code.difluir.com/
 */
if ( ! function_exists('difluir_custom_quote_shortcode') ) :
	function difluir_custom_quote_shortcode( $atts, $content = null ) {
		$atts = shortcode_atts( array(
	        'id' => ''
	    ), $atts, 'difluir_quote_shortcode' );
	    
	    if ( ! $content ) {
	    	return;
	    }

	    $id = ( isset($atts['id']) && !empty($atts['id']) ) ? ' id="'.esc_attr($atts['id']).'"' : '';

	    $output  = '<div'.$id.' class="difluir-custom-quote">';
	    	$output .= $content;
	    $output .= '</div>';

	    return $output;
	}
endif;

if ( ! shortcode_exists('difluir_quote_shortcode') ) {
	add_shortcode( 'difluir_quote_shortcode', 'difluir_custom_quote_shortcode' );
}

E onde quiser usar o shortcode, inserir assim:

[difluir_quote_shortcode id="quote"]
Um texto aqui
[/difluir_quote_shortcode]