add html attribute to checkbox element from php

I want to load a value (0/1) from a mysql database and then change a checkbox to "checked" according to the value obtained. I had a look at DOMElement but I get a blank page when adding the code. Here is the code I tried:

$html = '<input type="checkbox" name="alarm" value="1">';
$dom = new DOMDocument();
$dom->loadHTML($html);
foreach ($dom->getElementsByTagName('input') as $item)
{
    $item->setAttribute('checked', 'true');
    echo $dom->saveHTML();
    exit;
}

To be quite frank this means nothing to me but it looks as if it should work.. The setAttribute take $name and $value as paramaters, but a checkbox has only a attribute checked so it got me in a flat spin abit. Here is all my code. I'm open for any useful criticism, as I am a noob and still learning.. :)

PHP

$sqlSelect = "SELECT value FROM elements WHERE element='Alarm'";

$result = $conn->query($sqlSelect);

if ($result->num_rows > 0)
{
    while($row = $result->fetch_assoc())
    {
        echo "Value: " . $row["value"]. "<br>";
        if ($row["value"]) === 1)
        {
            $html = '<input type="checkbox" name="alarm" value="1">';
            $dom = new DOMDocument();
            $dom->loadHTML($html);
            foreach ($dom->getElementsByTagName('input') as $item)
            {
                $item->setAttribute('', 'checked');
                echo $dom->saveHTML();
                exit;
            }
        }
    }
}
else
{
     echo "0 results";
}

if(isset($_GET['alarm']))
    $alarm = 1;
else
$alarm = 0;

$sqlUpdate = "UPDATE elements SET value=" .$alarm. " WHERE element='alarm'";

if ($conn->query($sqlUpdate) === TRUE)
{
    echo "Record updated successfully";
}
else
{
    echo "Error updating record: " . $conn->error;
}

$conn->close();

HTML

<form id="alarm" action="test.php" method="get">
    <input type="checkbox" name="alarm" value="1" checked>Alarm
</form>

Please help this dumb fool...