wordpress源码:wp-blog-header

看wp源码时看到以下：

if ( !isset($wp_did_header) ) {

    $wp_did_header = true;

    require_once( dirname(__FILE__) . '/wp-load.php' );

    wp();

    require_once( ABSPATH . WPINC . '/template-loader.php' );

}

当请求开始时$wp_did_header永远都没有设置过，那么这个if语句就没有存在的必要了吧？

---

It's a global variable that can be checked by user functions and filters to see whether or not WordPress has already sent headers. It's just there to help.
-- http://wordpress.stackexchange.com/questions/20616/whats-the-purpose-of-wp-did-header

但是，我更赞同下面的解释：

对$wp_did_header进行赋值，这样如果代码块已经执行过，判断就会失败，代码块就不会再执行。这种做法可以确保wp-blog-header.php文件只执行一次（重复执行的话会出现函数名冲突、变量重置等，WordPress会精神分裂的！）；

-- http://www.ecdoer.com/post/wordpress-source-analysis.html

通过在源码中进行全局搜索，也可以看出：

$ grep wp_did_header ./ -nR
./wp-blog-header.php:8:if ( !isset($wp_did_header) ) {
./wp-blog-header.php:10:    $wp_did_header = true;
./wp-includes/template.php:495:    global $posts, $post, $wp_did_header, $wp_query, $wp_rewrite, $wpdb, $wp_version, $wp, $id, $comment, $user_ID;