WordPress 插件开发技巧：在上传目录下创建子目录

register_activation_hook( __FILE__, 'wpjam_plugin_activate' );
function wpjam_plugin_activate() {
	$upload = wp_upload_dir();
	$upload_dir = $upload['basedir'];
	$upload_dir = $upload_dir . '/wpjam-files';
	if (! is_dir($upload_dir)) {
		mkdir( $upload_dir, 0700 );
	}
}