PHP

            
<header>
    <div class="parallax-container">
      <?php $backgroundImg = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );?&<!-- get the thumbnail url -->
      <?php if (has_post_thumbnail()) { ?>
        <section class="header background-image" style="background-image: url('<?php echo $backgroundImg[0]; ?>');">
      <?php   } else { ?>
        <section class="header background-image" style="background-image: url('<?php echo get_template_directory_uri(); ?>/css/img/random/ran.svg')">
      <?php  }  ?> 
      <?php include('menu-top.php') ; ?><!-- Menu -->
        </section><!-- / Bannière -->
    </div>
</header>
                
<!-- alerte-->
<div class="buttons has-addons is-centered">
    <a class="button" href="tel:+33xxxxxxxxx" title="Communication directe">
        <span class="icon"><i class="fas fa-phone" aria-hidden="true" title="téléphone"></i></span>
        <span>05 49 00 00 00</span>
    </a>
    <p> Contactez-nous !</p>
</div>
<!-- / alerte-->
                
<main class="section">
    <div class="container">
        <div class="columns is-centered">
            <div class="column is-8">
                <?php echo the_breadcrumb() ?>
                <article>
                    <?php get_template_part('content') ?>
                </article>
            </div>
        </div>
    </div>
</main>
            
        

CSS-JS

            
                                        <!-- CSS -->

<style type="text/css">
    .background-image{
        background-image : url('https://images5.alphacoders.com/701/701401.jpg'); /*place holder*/
        background-size : cover;
        background-position : center;
    }
    .parallax-container{
        overflow: hidden;
    }
</style>
          

                                        <!-- JS -->

<script>
    window.addEventListener('scroll', () => {
        let scrolled = window.pageYOffset;
        let parallax = document.querySelector(".background-image");
        let coords = (scrolled * 0.4) + 'px';
            // effet ajustable en modifiant la valeur 0.x 
        parallax.style.transform = 'translateY(' + coords + ')';
    });
</script>