echo image caption

Retrieve caption of image correct format is

 <?php $post_thumbnail_id = get_post_thumbnail_id( $post_id ); ?> 

<?php

function wp_get_attachment( $attachment_id ) {

    $attachment = get_post( $attachment_id );
    return array(
        'alt' => get_post_meta( $attachment->ID, '_wp_attachment_image_alt', true ),
        'caption' => $attachment->post_excerpt,
        'description' => $attachment->post_content,
        'href' => get_permalink( $attachment->ID ),
        'src' => $attachment->guid,
        'title' => $attachment->post_title
    );
}

$at= wp_get_attachment($post_thumbnail_id);
print_r($at);//show array of all image detail like title , caption etc

?>