Why do PHP programs sometimes display at the beginning: x-powered-by:php/4.0.0 characters?

Program | shows this because of PHP. INI some of the settings are not correct, most likely is that there are invalid DLL files. But why is there such a thing?
I built an empty test.php file under the WIN32.
Then execute C:\php4\php.exe test.php, at which point my php.ini configuration is correct. The screen output is:
x-powered-by:php/4.0.0
Content-type:text/html
Gee, isn't that what's on the screen? Yes. However, these two lines are used as headers for HTTP messages. is not displayed in the browser.

Now I loaded the php_mysql.dll in the php.ini. And this file is not there.
To perform C:\php4\php.exe test.php again, the screen output is:
Content-type:text/html
PHP Fatal error:unable to load Dynamic Library ' Php_mysql.dll ' <br>
There are some random characters here.
x-powered-by:php/4.0.0
Content-type:text/html

That is, after content-type:text/html, there are some error messages, which causes x-powered-by:php/4.0.0 content-type:text/html to appear on the screen. And this is actually the HTTP header of the message.

Then I think that in the event of such a mistake, if the Output_buffering=off, then Setcookie () is no way to use.
Do you understand?