此处的答案假设用户具备以下知识

PHP，并知道如何从PHP联机手册中找到参考shortcode_atts() - 将用户属性与已知属性相结合，并在需要时填写默认值我想你没有用正确的形式写下这个短代码。

非法字符串偏移量“title”表示$atts[“title”]不存在。它不存在，因为在调用shortcode时，title 未给出

To resolve this, you need to set default values and then combine it with user attributes(the attributes supply when anyone call the shortcode).

add_shortcode( \'recent-blogs\', \'cp_sidebar_recent_blogs_shortcode\');
function cp_sidebar_recent_blogs_shortcode( $atts, $content = null ) {

    // the following is default value + merge value from shortcode calling, 
    // it can prevent errors in case any attribute is empty such as title is not defined
    extract(shortcode_atts(array(
        \'title\' => \'\',
        \'layout\' => \'\',
    ), $attr));

    // with extract() you don\'t need to explicitly create a $title_default
    // because extract() will create $title with value \'\'

    if ( $atts[\'title\'] == \'Related Posts\' && $atts[\'layout\'] == \'horizontal\' ) {
        $title = \'Related Posts\';
    }
    else {
        $title = \'Recent Posts\';
    }

    // ... your other code continue
}