你在看这一页。
this页面中需要注意的示例：

<?php
header("Location: http://www.example.com/"); /* Redirect browser */

/* Make sure that code below does not get executed when we redirect. */
exit;
?>

$_POST[password]="";

可能应该是（注意'）：

$_POST['password'] = "";

函数exit()在执行时肯定会停止执行，你的if条件肯定有问题。
在PHP中，多个值“等于”零（如果你使用==）。尝试var_dump($num)看看到底是什么。

exit无法工作，因为它具有2个依赖项
A.如果$num不为零，则if ($num == 0)退出不起作用
B. header("Location:index.php#captcha");如果您的位置工程退出不会wort
尝试

$num = mysql_num_rows ( $result );
if ($num == 0) {
    $_POST ['password'] = null;
    header ( "Location: http://yoursite.com/index.php#captcha" ); // Location:#errorlogin.html");
    exit();
}
else
{
    echo "Found $num in the database";
}

在RHEL/CentOS上将php版本切换到82后，exit（）和die（）停止工作。花了一天的大部分时间进行调试。结果发现'uopz'模块正在改变默认行为。

// PHP 7 
$ php -i | grep -A1 "uopz support"; php -r 'die; echo "Die did not work\n";'

// PHP 82
php -i | grep -A1 "uopz support"; php -r 'die; echo "Die did not work\n";'
uopz support => enabled
Version => 7.1.1

Die did not work

// Changed uopz.exit setting to 1 in /path/to/php.d/05-uopz.ini, to restore default die/exit behavior. Showing contents here:

; Enable 'User Operations for Zend' extension module
extension=uopz.so

; Configuration
; See http://php.net/manual/en/uopz.configuration.php

; If enabled, uopz should stop having any effect on the engine.
;uopz.disable = 0

; Whether to allow the execution of exit opcodes or not.
; This setting can be overridden during runtime by calling uopz_allow_exit().
;uopz.exit = 0
uopz.exit = 1

// Tested again - die worked:
php -i | grep -A1 "uopz support"; php -r 'die; echo "Die did not work\n";'
uopz support => enabled
Version => 7.1.1

对于一个Zend扩展来说，修改php的默认行为是一个令人震惊的发现。