Get first gallery shortcode WordPress

first-shortcode.php

<?php
function deo_get_attachment(){

  global $post;
  $pattern = get_shortcode_regex();  
  $output = '';

  if( preg_match_all( '/'. $pattern .'/s', $post->post_content, $matches )
    && array_key_exists( 2, $matches )
    && in_array( 'gallery', $matches[2] ) ):

    $keys = array_keys( $matches[2], 'gallery' );

    foreach( $keys as $key ):
      $atts = shortcode_parse_atts( $matches[3][0] );
      if( array_key_exists( 'ids', $atts ) ):

        $attachments = get_posts( array(
          'post_type' => 'attachment',
          'post_status' => 'inherit',
          'post__in' => explode( ',', $atts['ids'] ),
          'orderby' => 'post__in'
        ) );

      endif;
    endforeach;

    return $attachments;

    elseif( has_post_thumbnail() ):
      $output = wp_get_attachment_url( get_post_thumbnail_id( get_the_ID() ) );

  endif;
}