Storing PHP Arrays in WordPress User Meta Database

You need to set the last parameter from false to true:

$wp_user_film_prefs_arr = get_user_meta( $wp_user_id, $wp_user_film_prefs_key_label, true );

That third parameter is $single:

(boolean) (optional) If true return value of meta data field, if false return an array.

Default: false

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WordPress post featured image url

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

  </div>
<?php endif; ?>