关于PHP中Cann't redeclare class的错误

include.class.php
<?php

class IncludeTest {
}

?>

test.php
<?php
//报错
//Fatal error: Cannot redeclare class IncludeTest in D:\myphp\research\include_test\include.class.php on line 3
//require('include.class.php');
//require('include.class.php');

//报错
//Fatal error: Cannot redeclare class IncludeTest in D:\myphp\research\include_test\include.class.php on line 3
//require_once('include.class.php');
//require('include.class.php');

//不报错
//require('include.class.php');
//require_once('include.class.php');

//不报错
require_once('include.class.php');
require_once('include.class.php');

//总结，两次加载同一文件，只要后面一个用require_once，则不报错。
?>

还有一种情况，就是在多个地方加载了同一个类，但是即使用了incluce_once或require_once仍然会报错(暂时不知道是什么原因导致的)，此时有一种办法可以解决，那就是在加载之前用class_exists判断一下，且第二参数用false，这样可以提高性能，用class_exists判断一下后，就不会报Cann't redeclare class的错误了。

例如if (!class_exists('Smarty', false)) include_once('/path/to/Smarty.class.php');

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