PHP函数返回=“”而不是/

I have got a wordpress instance and i want to set background image inline a html tag via php function.

The html is:

<div id="content" class="site-content" <?php echo get_background_for_single(get_the_ID(), is_single()); ?>>

The above html and php has the following output in browser:

<div id="content" class="site-content" style="background-image: url(" http:="" localhost="" wiese="" wp-content="" uploads="" 2019="" 06="" test.jpg");="">

In the functions.php i got:

function get_background_for_single($ID, $single) {

    if ($single == 1) {
        return 'style="background-image: url("' . get_the_post_thumbnail_url($ID) . '");';
    }
}

If i controll the output in a paragraph as follows, it is displayed correct:

<p><?php echo get_background_for_single(get_the_ID(), is_single()); ?><p>

It returns in a p tag:

style="background-image: url("http://localhost/wiese/wp-content/uploads/2019/06/test.jpg");

Why does the function return " ="" " instead of " / "? Perhaps it is a dump mistake, but i cannot see my mistake.

Changing my function to the following is the solution:

function get_background_for_single($ID, $single) {

    if ($single == 1) {
        return 'style="background-image: url("' . get_the_post_thumbnail_url($ID) . '");"';
    }
}