WordPress If statement Feature Image

What I am trying to achieve is for posts that haven't set a featured image to use the default CSS background-image . That means every post will have an image.

<?php $url = wp_get_attachment_url( get_post_thumbnail_id($post->ID, 'thumbnail') ); ?>
 <div class="entry-header featureImg" style="background-image: url('<?php echo $url ?>')"></div>

Basically here is the thing :

<div class="entry-header featureImg" style="background-image: url('<?php echo $url ?>')">

for post without featured image set the 'background-image' should be the default one from CSS because as the code is actually written the bg image for those posts(whitout feat image set) is :

background-image: url(''); //taken from page source

this should be done with php if statements but I have no idea how to work with this syntax :

`<?php if($has_post_thumbnail) : ?>

  <?php $url = wp_get_attachment_url( get_post_thumbnail_id($post->ID,'thumbnail') ); ?>
<div class="entry-header featureImg" style="background-image: url('<?php echo $url ?>')">

<?php else : ?>

    <div class="entry-header featureImg">

<?php endif; ?>
?>`