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HomeBackend DevelopmentPHP TutorialPHP flush class output buffer analysis example code

该函数将当前为止程序的所有输出发送到用户的浏览器。

<?php 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
flush(); 
sleep(1); 
} 
?>

按照php手册里的说法
该函数将当前为止程序的所有输出发送到用户的浏览器。
上面的这段代码,应该隔一秒钟输出一次$i。但是实际中却不一定是这样。有可能是等了10秒钟后,所有的输出同时呈现出来。
好,我们来改一下这段代码,改成

<?php 
ob_end_clean
();//修改部分 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
flush(); 
sleep(1); 
} 
?>

嘿,加了这一句ob_end_clean();,居然就OK了。实际上,我们把ob_end_clean()换成ob_end_flush()也一样OK。
我再来改一改。

<?php 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
ob_flush
();//修改部分 
flush(); 
sleep(1); 
} 
?>

运行一下,是不是发现$i也隔一秒输出一次了?这是为什么呢?
别急,我们来看看php.ini。
打开php.ini,搜索output_buffering,我们会看到类似这样的设置 output_buffering = 4096。正如它的名字output_buffering一样,这个设置的作用就是把输出缓冲一下,缓冲大小为4096bytes.
在我们的第一段代码里,之所以没有按预期的输出,正是因为这个output_buffering把那些输出都缓冲了。没达到4096bytes或者脚本结束,输出是不会被发送出去的。
而第二段代码中的ob_end_clean()和ob_end_flush()的作用,就是终止缓冲。这样就不用等到有4096bytes的缓冲之后才被发送出去了。
第三段代码中,用了一句ob_flush(),它的作用就是把缓冲的数据发送出去,但是并不会终止缓冲,所以它必须在每次flush()前使用。
如果不想使用ob_end_clean(),ob_end_flush()和ob_flush(),我们就必须把php.ini里的 output_buffering设得足够小,例如设为0。需要注意的是,如果你打算在脚本中使用ini_set(” output_buffering”,”0″)来设置,那么请停下来吧,这种方法是不行的。因为在脚本一开始的时候,缓冲设置就已经被载入,然后缓冲就开始了。
可能你会问了,既然ob_flush()是把缓冲的数据发送出去,那么为什么还需要用flush()???直接用下面这段代码不行吗??

<?php 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
ob_flush(); 
sleep(1); 
} 
?>

请注意ob_flush()和flush()的区别。前者是把数据从PHP的缓冲中释放出来,后者是把不在缓冲中的或者说是被释放出来的数据发送到浏览器。所以当缓冲存在的时候,我们必须ob_flush()和flush()同时使用。
那是不是flush()在这里就是不可缺少的呢?不是的,我们还有另外一种方法,使得当有数据输出的时候,马上被发送到浏览器。下面这两段代码就是不需要使用flush()了。(当你把output_buffering设为0的时候,连ob_flush()和ob_end_clean()都不需要了)

<?php 
ob_implicit_flush
(true); 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
ob_flush(); 
sleep(1); 
} 
?> 
<?php 
ob_end_clean(); 
ob_implicit_flush(true); 
for ($i=10; $i>0; $i--) 
{ 
echo $i; 
sleep(1); 
} 
?>

请注意看上面的ob_implicit_flush(true),这个函数强制每当有输出的时候,即刻把输出发送到浏览器。这样就不需要每次输出(echo)后,都用flush()来发送到浏览器了。
以上所诉可能在某些浏览器中不成立。因为浏览器也有自己的规则。我是用Firefox1.5,IE6,opera8.5来测试的。其中opera就不能正常输出,因为它有一个规则,就是不遇到一个HTML标签,就绝对不输出,除非到脚本结束。而FireFox和IE还算比较正常的。
最后附上一段非常有趣的代码,作者为PuTTYshell。在一个脚本周期里,每次输出,都会把前一次的输出覆盖掉。
以下代码只在firefox下可用,其他浏览器并不支持multipart/x-mixed-replace的Content-Type.

<?php 
header(&#39;Content-type: multipart/x-mixed-replace;boundary=endofsection&#39;); 
print "\n--endofsection\n"; 
$pmt = array("-", "\\", "|", "/" ); 
for( $i = 0; $i <10; $i ++ ){ 
sleep(1); 
print "Content-type: text/plain\n\n"; 
print "Part $i\t".$pmt[$i % 4]; 
print "--endofsection\n"; 
ob_flush(); 
flush(); 
} 
print "Content-type: text/plain\n\n"; 
print "The end\n"; 
print "--endofsection--\n"; 
?>


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