I have PHP MySQL functions and I need to pass the id number from one function to another and each function are called in different page. I'm using the URL to pass the variable. I don't know if that is possible but can I use session to pass the id in a safe way so the users cannot manipulate the URL variable?

function 1 :

function select_all_majors() {
            global $connection;
    try {
        $output  = "";
        $output .= "<ul  class=\"\">";
        $sql = "SELECT * FROM `majors`";
        $statement = $connection -> prepare($sql);
        $statement -> execute();
        while ($results = $statement -> fetch(PDO::FETCH_ASSOC)) {
            $major_id   = $results["major_id"];
            $major_name = $results["major_name"];
            $output .= "<li>";
            $output .= "<a href=\"subjects.php?major_id = ";
            $output .= urlencode($major_id) ;
            $output .= "\">";
            $output .= htmlentities($major_name);
            $output .= "</a>";
            $output .= "</li>";
        }
        $output .= "<ul>";
             echo $output;
             return $results;
    } catch(PDOException $error) {

        $sql_error = $error -> getMessage();
        echo "<h4>" . $sql_error . "</h4>";
    }

}

function 2 :

function find_subjects_for_major() {

    global $connection ;
    //SELECT * FROM `subjects` WHERE `subjects`.`major_id` = 2
    try {       
        $output = "";
        $query = "SELECT * FROM `subjects`";
        $query.= " WHERE `subjects`.`major_id` = 2";
        $statement = $connection->prepare($query);
        $output  .= "<ul>";
        $statement->execute();
        while($results = $statement->fetch(PDO::FETCH_ASSOC)){

        $language_id   = $results["subject_id"];
        $language_name = $results["subject_name"];
        $output   .="<li>";
        $output   .="<a href =\"courses.php\">"; //?id={$language_id}//test.php?languege_id="{echo $result["language_id"];}
        $output   .= htmlentities($language_name)."</li>";
        $output   .="</a>";
        $output   .="</li>";
    }
    $output  .= "</ul>";
    echo $output ;
    return $results;

    } catch(PDOException $error) {
        $sql_error = $error -> getMessage();
        echo "<h4>" . $sql_error . "</h4>";
    }
}