Why do I get undefined index in this code?

Question

$a=filter_input(INPUT_POST,'submit');
if(isset($a)) { $myFile=$_FILES['M']['name'];
$name=$_FILES['M']['tmp_name'];
if(isset($myFile)) {
if(!empty($myFile)) {echo 'success';}
}else{echo 'select a picture; }

I used netbeans editor

Answer 1

in the first line use

$_POST['submit']