Undesired empty rows in html source

Using php to generate automatically a webpage can produce undesired empty rows (typically at the begging of your source code, before <!DOCTYPE html>).
To avoid them you could have i.g. to remove empty rows between ?> and <?php, that is something like the follow code:

?>

<?php

The (new) working code should be

?>
<?php

On the contrary, empty rows inside <?php ... ?> blocks are not relevant.

In general, every empty (useless) row after a ?> could generate undesired empty rows in your html output source code.