javascript issue

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>Test</title>
</head>
<body>
<?php

$day = range(1,31);
$month = range(1,12);
$year = range(2019,2040);

echo"<br />\n";
echo "Select Date (d/m/y)&nbsp;";
echo "<select name=\"selectDay\" id=\"selectDay\">\n";
foreach($day as $key =>$value){
        $day_value = str_pad($value, 2, '0', STR_PAD_LEFT);
        echo "<option value='$day_value'>$day_value</option>\n";
        }
        $day_value = str_pad($day[0], 2, '0', STR_PAD_LEFT);
        echo "<option value=$day_value selected='selected'>$day_value</option></select>\n";
//
echo "<select name=\"selectMonth\" id=\"selectMonth\">\n";
foreach($month as $key =>$mvalue){
        $month_value = str_pad($mvalue, 2, '0', STR_PAD_LEFT);
        echo "<option value='$month_value'>$month_value</option>\n";
        }
        $month_value = str_pad($month[0], 2, '0', STR_PAD_LEFT);
        echo "<option value=$month_value selected='selected'>$month_value</option></select>\n";
//
echo "<select name=\"selectYear\" id=\"selectYear\">\n";
foreach($year as $key =>$value){
        echo "<option value='$value'>$value</option>\n";
        }
        }
        echo "<option value={$year[0]} selected='selected'>{$year[0]}</option></select>\n";
//
?>
<button onclick="myFunction()">Submit</button>

<p id="demo"></p>

<script>
function myFunction() {
        var x = document.getElementById("selectDay").value;
        var y = document.getElementById("selectMonth").value;
        var z = document.getElementById("selectYear").value;
        document.getElementById("demo").innerHTML =
        "Date selected = " + x + '-' + y + '-' + z;
}
// SOLUTION - hyphens needed to be quoted above
</script>
</body>
</html>