php include 和 require区别

刚接触php不久，碰到include和require, 在网上查了很久，很多人也做了比较，有些不大正确，有误导之嫌，最权威的正确的答案在这里：http://cn.php.net/manual/zh/function.include.php。
看了如果还不明白的话，我做了以下测试，
情况一：
<?php
include("./est.php"); // est.php不存在
echo "<br>continue...";
?>

测试结果：
Warning: include(./est.php): failed to open stream: No such file or directory in .../test1.php on line 2 Warning: include(): Failed opening './est.php' for inclusion (include_path='.:/usr/local/Cellar/php/5.3.10/lib/php') in .../test1.php on line 2
continue...

情况二：
<?php
require("./est.php");
echo "<br>continue...";
?>

测试结果：
Warning: require(./est.php): failed to open stream: No such file or directory in .../test1.php on line 2 Fatal error: require(): Failed opening required './est.php' (include_path='.:/usr/local/Cellar/php/5.3.10/lib/php') in .../test1.php on line 2
此处continue字符串没有打印出来。