WordPress, PHP, Get lazyload friendly attachement image based on attachment ID

lazyload-attachment.php

/**
 * Gets an attachement image as lazy loaded img based on ID
 * @param  string  $size          Thumbnail, medium, large, original, or {custom size}
 * @param  string  $class         CSS class name for the img tag
 * @param  integer $attachment_id ID of the attachment
 * @param  string  $attachment_id The custom src atrribute to use, default to data-echo
 * @return string                 Lazyload friendly HTML img tag
 */
function get_attachemt_lazy($size, $class, $attachment_id = "", $src_attr = "data-echo")
{
    $src = wp_get_attachment_url($attachment_id);
    $attr = array(
        'class' => "attachment-$size $class",
        'src' => 'data:image/gif;base64,R0lGODlhAQABAIAAAMLCwgAAACH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==',
        $src_attr => $src,
    );
    $output = wp_get_attachment_image($attachment_id, $size, false, $attr);
    return $output;
}