Header () and session_start () can not have any output in front of what is meant, how to solve

Header () and session_start () can not have any output in front of what meaning
This is a function in common.php: [Code=php][/code]function checkuservalidate () {
Session_Start ();
if (Empty ($_session[' Loginuser ')) {
Header ("Location:login.php?errno=1");
}
}[/code]

PHP Code

Why require_once ' common.php ';
Checkuservalidate (); not already in the forefront of the PHP code, there is no output in front of it? Why is it wrong?
Be sure to write a snippet of code before the HTML tag contains these two words?

------Solution--------------------
The ultimate goal in the front is to prevent early output.
------Solution--------------------
Get to know the HTTP protocol.
------Solution--------------------
Cache overflow

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