Why is SQL injection dangerous?

Question

Database

Why is SQL injection dangerous?
How can they be bypassed?
What can be done instead of them?

// Получение одной статьи по её id
function get_lesson_by_id($id) {
    global $db;
    $bookmarks = $db->query("SELECT * FROM entries WHERE id = $id");
    foreach ($bookmarks as $bookmark) {
    return $bookmark;
    }
}

Answer 1