php在函数外宣言global变量有什么用

php在函数外声明global变量有什么用？
php在函数外声明global变量有什么用？

我看wordpress的wp-settings.php文件中有这么一段：

/*<br />
 * These can't be directly globalized in version.php. When updating,<br />
 * we're including version.php from another install and don't want<br />
 * these values to be overridden if already set.<br />
 */<br />
global $wp_version, $wp_db_version, $tinymce_version, $required_php_version, $required_mysql_version;<br />
require( ABSPATH . WPINC . '/version.php' );

不懂，求解释，谢谢啦 ^_^没有作用
并且在 wp-settings.php 中也没有看到
global $wp_version, $wp_db_version, $tinymce_version, $required_php_version, $required_mysql_version;
只有
require( ABSPATH . WPINC . '/version.php' );没作用