如何用php脚本 判断客户端浏览器是否已经断开连接？
曾经看到一个人贴：
在 PHP 内部，系统维护着连接状态，其状态有三种可能的情况：
0 - NORMAL（正常）
1 - ABORTED（退出）
2 - TIMEOUT（超时）
当 PHP 脚本正常地运行 NORMAL 状态时，连接为有效。当远程客户端中断连接时，ABORTED 状态的标记将会被打开。远程客户端连接的中断通常是由用户点击 STOP 按钮导致的。当连接时间超过 PHP 的时限（请参阅 set_time_limit() 函数）时，TIMEOUT 状态的标记将被打开。
您可以决定您的脚本是否需要在客户端中断连接时退出。有时候让您的脚本完整的运行会带来很多方便，即时没有远程浏览器接受脚本的输出。默认的情况是当远程客户端连接中断时脚本将会退出。该处理过程可由 php.ini 的 ignore_user_abort 或由 Apache .conf 设置中对应的“php_value ignore_user_abort”以及 ignore_user_abort() 函数来控制。如果您没有告诉 PHP 忽略用户的中断，您的脚本将会被中断，除非您通过 register_shutdown_function() 设置了关闭触发函数。通过该关闭出发函数，当远程用户点击 STOP 按钮后，您的脚本再次尝试输出数据时，PHP 将会检测到连接已被中断，并调用关闭触发函数。
您的脚本也有可能被内置的脚本计时器中断。默认得超时限制为 30 秒。这个值可以通过设置 php.ini 的 max_execution_time 或 Apache.conf 设置中对应的“php_value max_execution_time”参数或者 set_time_limit() 函数来更改。当计数器超时候，脚本将会类似于以上连接中断的情况退出，先前被注册过的关闭触发函数也将在这时被执行。在该关闭触发函数中，您可以通过调用 connection_status() 函数来检查超时是否导致关闭触发函数被调用。如果超时导致了关闭触发函数的调用，该函数将返回 2。
需要注意的一点是 ABORTED 和 TIMEOUT 状态可以同时有效。这在你告诉 PHP 忽略用户的退出操作时是可能的。PHP 将仍然注意用户已经中断了连接但脚本仍然在运行的情况。如果到了运行的时间限制，脚本将被退出，设置过的关闭触发函数也将被执行。在这时您会发现函数 connection_status() 返回 3。
但是我测试的结果是，如果后端没有向前端输出的话，比如在一个死循环的While处理中，没有向前端输出内容，那connection_status()永远都是0，根本不知道前端断开了。
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