Eureka, I found a solution, here it is!
<?php
$kommatest = $_POST[kortenaam];
$newtest = str_replace("'","’",$kommatest);
$conn=odbc_connect('starmakertest2','','');
if (!$conn)
{exit("Connection Failed: " . $conn);}
$sql="UPDATE starmaker1test SET ID = '$newtest' WHERE Id1 = $_POST[idnummer]";
$rs=odbc_exec($conn,$sql);
if (!$rs)
{exit("Error in SQL");}
echo "<b>$_POST[kortenaam]</b><br/>";
?>