array ( 0 => 'index.php', 1 => 'PHP Manual', ), 'head' => array ( 0 => 'UTF-8', 1 => 'zh', ), 'this' => array ( 0 => 'function.require-once.php', 1 => 'require_once', 2 => 'require_once', ), 'up' => array ( 0 => 'language.control-structures.php', 1 => '流程控制', ), 'prev' => array ( 0 => 'function.include.php', 1 => 'include', ), 'next' => array ( 0 => 'function.include-once.php', 1 => 'include_once', ), 'alternatives' => array ( ), 'source' => array ( 'lang' => 'zh', 'path' => 'language/control-structures/require-once.xml', ), 'history' => array ( ), ); $setup["toc"] = $TOC; $setup["toc_deprecated"] = $TOC_DEPRECATED; $setup["parents"] = $PARENTS; manual_setup($setup); contributors($setup); ?>

require_once

(PHP 4, PHP 5, PHP 7, PHP 8)

require_once 表达式和 require 表达式完全相同，唯一区别是 PHP 会检查该文件是否已经被包含过，如果是则不会再次包含。

参见 include_once 的文档来理解 _once 的含义，并理解与没有 _once 时候有什么不同。