$script = 'SELECT foo FROM bar WHERE item1 = ' . $item1;
if(defined($item2)) { $script .= ' and item2 = ' . $item2; }
if(defined($item3)) { $script .= ' and item3 = ' . $item3; }


##</code><code>##

$script = 'SELECT foo FROM bar WHERE item1 = ?';
if(defined($item2)) { $script .= ' and item2 = ?';
if(defined($item3)) { $script .= ' and item2 = ?';

##</code><code>##

$sql->execute($item1,$item2,$item3);