css3 modal in php custom code

hello,

how can i insert a modal window in custom code php (this is my simple code, just message after upload):

if ($nomdupdf=="") {
echo "<font color =red><h2>La commande n'a pas été enregistrée</h2></font>";

this message say if field is empty then the formular is not send.
is it possible to insert css3 modal ?

thank's

Jl