Hey @Ruriko ... its a 3 step action.

获取当前类别ID获取此特定类别的帖子将帖子放入选择菜单请注意，此代码将使用第一个类别ID，尽管您可以为同一帖子分配多个类别。。

<?php 
    // FIRST CATEGORY NAME
    $category = get_the_category(); 
    $catID = $category[0]->term_id;
    $args = array(
        \'numberposts\' => 5, 
        \'category\' => $catID            
    );
    $catPosts = get_posts( $args );

    echo \'<form method="POST">\';
    echo \'<select name="goToPost" onchange="document.location=this.value">\';
    echo \'<option value="">\'.__(\'Relevent Posts\', \'your_text_domain\').\'</option>\';  
    foreach( $catPosts as $singlePost ) {
    echo \'<option value="\'.get_bloginfo(\'url\').\'/index.php?p=\'.$singlePost->ID.\'">\'.$singlePost->post_title.\'</option>\';
    };
    echo \'</select>\';
    echo \'</form>\';
?>