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function my_search_form( $form ) {
$form = '<form role="search" method="get" id="searchform" action="'.home_url( '/' ).'" >
<input type="text" value="' . get_search_query() . '" name="s" id="s" placeholder="サイト内検索"/>
<input type="submit" id="searchsubmit" value="[" />
</div>
</form>';
return $form;
}

add_filter( 'get_search_form', 'my_search_form' );

function my_posy_search($search) {
  if(is_search()) {
    $search .= " AND post_type = 'post'";
  }
  return $search;
}
add_filter('posts_search', 'my_posy_search');