PHP $_POST Variable with brackets

Using array notation in forms with a PHP backend

<textarea name="ii_pi_name[]" value="<?= $ii_pi_name1 ?>" ></textarea>

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While array notation is useful to process forms - you need to use it in the right way. The above is useful if you have many textarea controls that you need to group together.

If you have just one then an open array is not really that useful.
It would be better to do something like this

<textarea name="data[pi_name]" value="<?= $ii_pi_name1 ?>" ></textarea>

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What we are doing here is using data as the container for the form data and then using the index name as the field.

In this way we can do this
$data = isset($__POST['data']) ? $_POST['data'] : false;

if ($data) {
   echo "Textarea value = {$data['pi_name']}";
}

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Going back to your implementation you could do this

$data = isset($__POST['data']) ? $_POST['data'] : false;

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Same as before except now your data is available only through its numeric index in the array. If we dump the above (assuming no other fields) we get this

Array (
   0 => 'Value from textarea'
)

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This is not that useful - because if you add other elements before the text area for instance if you add this to the front of your form

<input type="text" name="ii_pi_name[]">

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Now the dump of $data gives you

Array (
  0 => 'input value',
  1 => 'textarea value'
)

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It is difficult to make a proper recommendation but on the balance I would personally go with the named index for the array data.