Why sometimes PHP Programs Display X-Powered-ByPH at the beginning?

This is because of PHP. Some settings in INI are incorrect. The most likely reason is that there are invalid DLL files. But why?
I created an empty test. php file under WIN32.
Then execute c: php4php.exe test. php. At this time, my php. ini configuration is correct. Screen output:
X-Powered-By: PHP/4.0.0
Content-type: text/html
Isn't that something displayed on the screen? Yes. However, these two lines are used as the HTTP information header. Is not displayed in the browser.
Now I have installed php_mysql.dll in php. ini. This file does not exist.
Run c: php4php.exe test. php again, and the screen output is:
Content-type: text/html
PHP Fatal error: Unable to load dynamic library 'php _ mysql. dll'
There are still some characters in disorder
X-Powered-By: PHP/4.0.0
Content-type: text/html
Some error messages are generated after Content-type: text/html, which leads to X-Powered-By: PHP/4.0.0 Content-type: text/html appears on the screen. This is actually the HTTP information header.
In this case, if output_buffering is off, setcookie () cannot be used.
Do you understand?