您正在尝试连接foreach 语句转换为字符串，这无法完成。。。您需要将所需的输出添加到现有字符串中，而不是连接它

function create_galeri_shortcode( $atts ) {

    $atts = shortcode_atts(
        array(
        ),
        $atts,
        \'galeri\'
    );

    if ( has_post_format( \'gallery\' ) ) {
        $images = get_post_meta( get_the_ID(), \'vdw_gallery_id\', true );
        if( $images ) {
            $out = \'<div id="lightgallery">\'.
            foreach ($images as $imageid) {
                $altyazi = get_post_meta( get_the_ID(), \'_wp_attachment_image_alt\', true );
                $out .= \'<a href="\'.wp_get_attachment_url($imageid, \'large\').\'" data-sub-html=".caption">\';
                    $out .= \'<img src="\'.wp_get_attachment_url($imageid, \'thumbnail\').\'" alt="\'.$altyazi.\'">\';
                    $out .= \'<div class="caption">\';
                        $out .= \'<h4>\'.$altyazi.\'</h4>\'; 
                        $out .= \'<p>\'.wp_get_attachment_caption( $imageid ).\'</p>\';
                    $out .= \'</div>\';
                $out .= \'</a>\';
            }
            $out .= \'</div>\';
        }
    }
return $out;
}
add_shortcode( \'galeri_mm\', \'create_galeri_shortcode\' );