When you add []
to the end of an input name, that causes all the values of those inputs to be put into an array in PHP. So $_POST['checked_id']
is an array, not a string. Then when you do:
value="<?= $_POST['checked_id'] ?>"
you're trying to echo the entire array into that one input. You can't echo an array, so you get that warning.
If this is part of a loop, you need to index the array
for ($i = 0; $i < count($_POST['checked_id']); $i++) { ?>
...
<input type="hidden" name="checked_id[]" value="<?=$_POST['checked_id'][$i]?>">
...
<?php
}