I have a form A that sends me to a page with a form B (I get the data of A with the post method), the submit of B makes modifications in a database and then I have to return to B but obviously when putting header ('Location: B.php') I get an error because I'm not sending data from A
You should be more specific to better understand your problem and give you a better answer.
What kind of data are you sending from form A to B and from B to php? Why POST?
You could try using Ajax instead of going from page to page and then going back.
But if something is useful, you can also try JavaScript
<?php
#Tu codigo aqui
#Una vez finalizado en vez de usar location(); intenta lo siguiente
?>
<script>
(function(){
window.history.back();
})()
</script>