CREATE TABLE klicks (
datum DATE NOT NULL PRIMARY KEY,
klicks INTEGER);

$sql = "SELECT count(*) 
FROM klicks
WHERE datum = current_date()";
$res = mysql_query($sql) or die(mysql_error());
list($count) = mysql_fetch_row($res);
if ($count)
{
    $sql = "UPDATE klicks
        SET klicks = klicks + 1
        WHERE datum = current_date()";
}
else
{
    $sql  = "INSERT INTO klicks (datum, klicks)
        VALUES (current_date(), 1);
} 
mysql_query($sql) or die(mysql_error());