Need assistamce with "Use of undefined constant ProductID - assumed 'ProductID'"

I have only one field "ProductID" in a temporary table.  The field is unique.  I am attempting to extract the ProductID with a select statement and I am receiving a error:

"Use of undefined constant ProductID - assumed 'ProductID'"

The ProductID will be included in a URL string to open individual product pages and scrape data.

$query1 = $conn->query('SELECT ProductID FROM urldownload');

while ($row1 = $query1->fetch(PDO::FETCH_ASSOC))
{

echo $row1[ProductID]."<BR>";


}		

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