PHP包含了以php://開頭的一系列輸出輸出流，如php://stdin, php://stdout等。今天查看代碼時(shí)，忽然想到一個(gè)問題：php://output和php://stdout有什么區(qū)別？

從PHP的官方文獻(xiàn)中找答案，對(duì)輸入流php://stdin和php://input的解釋分別如下（輸出流的解釋過于簡(jiǎn)略）：

php://stdin

php://stdin, php://stdout and php://stderr allow direct access to the corresponding input or output stream of the PHP process. The stream references a duplicate file descriptor, so if you open php://stdin and later close it, you close only your copy of the descriptor-the actual stream referenced by STDIN is unaffected. Note that PHP exhibited buggy behavior in this regard until PHP 5.2.1. It is recommended that you simply use the constants STDIN, STDOUT and STDERR instead of manually opening streams using these wrappers.

php://stdin is read-only, whereas php://stdout and php://stderr are write-only.

php://input

php://input is a read-only stream that allows you to read raw data from the request body. In the case of POST requests, it is preferable to use php://input instead of $HTTP_RAW_POST_DATA as it does not depend on special php.ini directives. Moreover, for those cases where $HTTP_RAW_POST_DATA is not populated by default, it is a potentially less memory intensive alternative to activating always_populate_raw_post_data. php://input is not available with enctype=”multipart/form-data”.

文檔并未直接闡述兩者的區(qū)別，仔細(xì)對(duì)比可得出以下信息：1. 均是只讀流； 2. php://stdin是PHP進(jìn)程的標(biāo)準(zhǔn)輸入，php://input用來讀取請(qǐng)求正文的原始數(shù)據(jù)。通過這些信息，該如何正確認(rèn)識(shí)兩者的本質(zhì)區(qū)別？

順著php://stdin進(jìn)程輸入的提示，聯(lián)想PHP進(jìn)程的執(zhí)行過程，再結(jié)合SAPI的差異，可以得到兩者主要區(qū)別：php://stdin是PHP進(jìn)程的輸入流，執(zhí)行生命周期內(nèi)均可能有數(shù)據(jù)流入（例如CLI下的交互式輸入）；php://input是PHP執(zhí)行時(shí)的外部輸入流，一般數(shù)據(jù)只能讀一次（具體看SAPI的實(shí)現(xiàn)）。同理可得到php://stdout和php://output的區(qū)別：php://stdout是PHP進(jìn)程的標(biāo)準(zhǔn)輸出流，php://output是返回的結(jié)果數(shù)據(jù)流。

下面用代碼驗(yàn)證結(jié)論：

// file: test.php
file_put_contents("php://output", "message sent by output" . PHP_EOL);
file_put_contents("php://stdout", "message sent by stdout" . PHP_EOL);
print("message sent by print" . PHP_EOL);
 
echo "SAPI:" , PHP_SAPI , PHP_EOL;

命令行執(zhí)行文件，輸出如下：

message sent by output
message sent by stdout
message sent by print
SAPI:cli

瀏覽器端請(qǐng)求，輸出如下：

message sent by output
message sent by print
SAPI:fpm-fcgi

在命令行下，PHP進(jìn)程的標(biāo)準(zhǔn)輸出流和結(jié)果輸出流均指向終端，所有消息都打印出來。在瀏覽器端，PHP進(jìn)程的輸出流被忽略，只有結(jié)果數(shù)據(jù)流被發(fā)送到web服務(wù)器。同時(shí)，print和echo調(diào)用的信息都作為執(zhí)行結(jié)果發(fā)往結(jié)果輸出流，所以都正常顯示。

最后再感慨一下PHP內(nèi)置函數(shù)的簡(jiǎn)潔實(shí)用，一個(gè)file_put_contents函數(shù)就搞定流寫入操作，換Java需要stream/writer一堆代碼，也省去C風(fēng)格的fopen/fwrite/fclose的繁瑣。

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