It is necessary to be noted that stream_set_blocking() and stream_set_timeout() does not work width standard I/O streams, such as STDIN and STDOUT.
(PHP 4 >= 4.3.0, PHP 5, PHP 7, PHP 8)
stream_set_blocking — 为资源流设置阻塞或者阻塞模式
$stream
, bool $enable
) : bool
为 stream 设置阻塞或者非阻塞模式。
此函数适用于支持非阻塞模式的任何资源流（常规文件，套接字资源流等）。
成功时返回 true， 或者在失败时返回 false。
Note:
在 Windows 系统上，这对本地文件没有影响。Windows 不支持本地文件的非阻塞 IO。
It is necessary to be noted that stream_set_blocking() and stream_set_timeout() does not work width standard I/O streams, such as STDIN and STDOUT.
On Windows this function does not work with pipes opened with proc_open (https://bugs.php.net/bug.php?id=47918, https://bugs.php.net/bug.php?id=34972, https://bugs.php.net/bug.php?id=51800)
When you use fwrite() on a non-blocking stream, data isn't discarded silently as t dot starling said.
Remember that fwrite() returns an int, and this int represents the amount of data really written to the stream. So, if you see that fwrite() returns less than the amount of written data, it means you'll have to call fwrite() again in the future to write the remaining amount of data.
You can use stream_select() to wait for the stream to be available for writing, then continue writing data to the stream.
Non-blocking streams are useful as you can have more than one non-blocking stream, and wait for them to be available for writing.