PHP: cannot set a cookie

Question

I want to set a cookie but it wont work. Im pretty new in PHP but I searched on the internet how to set a cookie, but it doesn't work.

<div id="content">
<?php
    if(isset($_GET['search-item']) && !isset($_COOKIE["searchRequest"])) {
        setcookie("searchRequest", $_GET['search-item']);
        include 'search_engine.php';
    }
    else {
        $page = $_COOKIE['page'];
        if(isset($page)){
            require_once $page;
        }
        else {
            require_once 'startpage.php';
        }
    }

?>
</div>

I really dont know what Im doing wrong :( Can someone help me and explain why this wont work. Everything else works flawlessly

Answer 1

First enable error reporting:

error_reporting(E_ALL);
ini_set('display_errors', 1);

then you should see following:

 PHP Warning: Cannot modify header information - headers already sent 

Because you cannot output anything before setcookie, and you do here:

<div id="content">

Manual says:

Like other headers, cookies must be sent before any output from your script (this is a protocol restriction). This requires that you place calls to this function prior to any output, including and tags as well as any whitespace.

Just move it after you set a cookie:

<?php
    if(isset($_GET['search-item']) && !isset($_COOKIE["searchRequest"])) {
        setcookie("searchRequest", $_GET['search-item']);
        echo '<div id="content">';
        include 'search_engine.php';
    }else{
        echo '<div id="content">';
        if(isset($_COOKIE['page'])){
            $page = $_COOKIE['page'];
            require_once $page;
        }else{
            require_once 'startpage.php';
        }
    }
    echo '</div>';
?>

And beware : it is not safe to include file that comes from cookie, I hope you just experiment, and it is not a part of serious code.