<p class="alert-danger"><strong>Achtung!</strong> This alert box indicates a dangerous or potentially negative action.<span class="alert-danger-closebtn" onclick="this.parentElement.style.display='none';">×</span></p>
so I have created my own alert box but the problem is that it doesn't work with php, can you help me?
Please take some time to learn PHP syntax, this is a pretty obvious solution. You need to escape or change the quotes.
Escaping:
echo "<p class=\"alert-danger\"><strong>DANGER!</strong> This alert box indicates a dangerous or potentially negative action.<span class=\"alert-danger-closebtn\" onclick=\"this.parentElement.style.display='none';\">×</span></p>";
Change the quotes (have to escape the single quotes still):
echo '<p class="alert-danger"><strong>DANGER!</strong> This alert box indicates a dangerous or potentially negative action.<span class="alert-danger-closebtn" onclick="this.parentElement.style.display=\'none\';">×</span></p>';