How to fix error Cannot modify header information?

<?php
 require_once "/lib/functions.php";
 if (!((checkUser($_SESSION["login"], $_SESSION["password"])) && (isAdmin($_SESSION["login"])))) {
 header("Location: login.php");
exit;
}
?>
June 26th 19 at 14:20
4 answers
June 26th 19 at 14:22
It seems the owner of the domain, notably burnt from such issues. - laila.Zbonc commented on June 26th 19 at 14:25
June 26th 19 at 14:24
There may be a output file require_once "/lib/functions.php";
June 26th 19 at 14:26
1. forget once and forever about the closing tag ?>. use only if you yuzaete PHP as template engine
2. ob_* functions
June 26th 19 at 14:28
ob_start();

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