Why is clicking on a link necessary for the $_SERVER['HTTP_REFERER'] variable to contain data?

When a user clicks on a link to navigate from one page to another, the HTTP_REFERER variable in the $_SERVER superglobal is populated with the URL of the previous page. If a user directly types in a URL or accesses a page through a bookmark, the HTTP_REFERER variable will be empty. To ensure that the HTTP_REFERER variable contains data, users must navigate through links on the website. 

// Check if the HTTP_REFERER variable is set and not empty
if(isset($_SERVER['HTTP_REFERER']) && !empty($_SERVER['HTTP_REFERER'])){
    // Use the HTTP_REFERER variable for further processing
    $previousPage = $_SERVER['HTTP_REFERER'];
    echo "Previous Page: " . $previousPage;
} else {
    echo "HTTP_REFERER variable is empty.";
}

Keywords

$_SERVER HTTP_REFERER link data clicking

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