You either need to [man]echo[/man] the values within the if/else blocks, or else escape out of PHP mode:
<?php
// echo method:
if ($flexplek_nieuw == '')
{
echo "<input type='text' name='required' size='9'></td>\n";
}
// escape out of PHP method:
else
{
?>
<input type='text' name='not_required' size='9'></td>;
<?php
}
?>