Wordpress和PHP检查自定义字段是否存在，如果不是随机字符串替换

I'm trying to create a function that checks for the existence of a post_thumbnail() for a post in Wordpress and sets an element's background image appropriately. Here's what I've tried:

 <div class="header-cover-image" style="background-image: url(
    <?php if ( has_post_thumbnail() ) {
      the_post_thumbnail('full');
    } else { echo get_template_directory_uri() . '/assets/img/article-overlay-1.jpg' }?>
    ">
  </div>

This currently crashes/causes the page to render nothing at all. I'm so confused about when and where to use quotation marks when using PHP within HTML markup!

Any ideas how I can make this work?

There was a problem with trying to echo the Wordpress function and append the string simultaneously. Fixed it by doing:

<?php if ( has_post_thumbnail() ) {
  the_post_thumbnail('full');
} else { $uri = get_template_directory_uri(); echo $uri . '/assets/img/article-overlay-1.jpg'; }?>
">

Try bloginfo ;

<?php if ( has_post_thumbnail() ) {
  the_post_thumbnail('full');
} else { echo get_bloginfo('stylesheet_directory') . '/assets/img/article-overlay-1.jpg'; }?>

Cheers!