get featured image id wordpress

1<?php if (has_post_thumbnail( $post->ID ) ): ?>
2  <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-thumbnail' ); ?>
3  <div id="custom-bg" style="background-image: url('<?php echo $image[0]; ?>')">
4
5  </div>
6<?php endif; ?>

1<?php echo the_post_thumbnail(); ?>

1get_post_thumbnail_id($post) 

1<?php if(has_post_thumbnail()){
2  the_post_thumnbail('cstm_name', array( 'class' => 'class1 class2' ))
3} else { ?>
4  <img class="card-img-top" src="http://placehold.it/750x300" alt="Card image cap" class="img-responsive">
5<?php } ?>