Why use foreach to get attachment_id if numberposts is equal to 1?

Question

I needed to pass an image to a Pinterest plugin in posts without a Featured Image so I wrote a little function to get the id of the first picture in a post. This code is in the loop-single.php where the sharing buttons are located.

if ( has_post_thumbnail()) {
    $thumb_for_pinterest = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'full' );
} else {
    $attachments = get_children( array(
        'post_parent'    => get_the_ID(),
        'post_type'      => 'attachment',
        'numberposts'    => 1, // show all -1
        'post_status'    => 'inherit',
        'post_mime_type' => 'image',
        'order'          => 'ASC',
        'orderby'        => 'menu_order ASC'
    ) );

    foreach ( $attachments as $attachment_id => $attachment ) {
        $thumb_for_pinterest = wp_get_attachment_image_src($attachment_id, 'full' );
    }
}

My question is this: why do I need to use a foreach to get the attachment_id?
Specifically, what is this line doing?

foreach ( $attachments as $attachment_id => $attachment ) {...

I believe an associative array is being returned with only 1 object in it. However, when I tried to just pop the object out of the array and use it I got lots of errors. Only by putting it in the foreach could I get the image id out.

Is there an easier way to get the attachment_id of the first image in a post?

I have read a lot of questions and googled this quite extensively, but couldn't understand it completely.

Answer 1

get_children will return

an associative array of posts [...] with post IDs as array keys, or an empty array if no posts are found.

---

This statement $attachment_id => $attachment is the same as the more commonly used $key => $value.

Using this command echo '<pre>'.print_r($attachments,true).'</pre>'; we get the following:
^{resumed version of a get_children with numberposts => 2}

Array
(
    [83] => stdClass Object
        (
            [ID] => 83
            [post_author] => 1
            [post_date] => 2012-06-22 05:06:22
            [post_content] => 
            [post_title] => barajas
            [post_excerpt] => 
            [post_status] => inherit
            [post_name] => barajas-4
            [post_modified] => 2012-06-22 05:06:22
            [post_modified_gmt] => 2012-06-22 05:06:22
            [post_parent] => 28
            [guid] => http://wp00.dev/wp-content/uploads/barajas3.jpg
            [menu_order] => 0
            [post_type] => attachment
            [post_mime_type] => image/jpeg
        )

    [180] => stdClass Object
        (
            [ID] => 180
            [post_author] => 1
            [post_date] => 2012-06-30 04:18:06
            [post_content] => 
            [post_title] => Yuri Atom_00281
            [post_excerpt] => 
            [post_status] => inherit
            [post_name] => yuri-atom_00281
            [post_modified] => 2012-06-30 04:18:06
            [post_parent] => 28
            [guid] => http://wp00.dev/wp-content/uploads/Yuri-Atom_00281.jpg
            [menu_order] => 0
            [post_type] => attachment
            [post_mime_type] => image/jpeg
        )

)

So, [83] and [180] are the $key and stdClass Object is their $value.
I think when calling get_children a foreach is almost mandatory, and this will be clear next...

---

If you use get_posts with exactly the same arguments, the result will be exactly the same except for one difference, the returned array will have numeric sequential keys:

Array
(
    [0] => stdClass Object
        (
            [ID] => 83
            [post_author] => 1
            [post_date] => 2012-06-22 05:06:22
        )

    [1] => stdClass Object
        (
            [ID] => 180
            [post_author] => 1
            [post_date] => 2012-06-30 04:18:06
        )
)

---

All that being said, if you use get_posts and numberposts => 1, you can substitue the foreach block for:

$thumb_for_pinterest = wp_get_attachment_image_src($attachments[0]->ID, 'full' );

---

With get_children we don't know what's the $key number, as it is the same as the Attachment ID, hence the foreach...

But with a regular numeric key (0, 1, 2, 3... etc), you can drop the foreach and call the first element of the array directly ($attachments[0]) and its object element (->ID).