How to pass/get placeholder value? [PHP] : So I have a form where one entry is:

    <input type="text" id="ptid" name="ptid" readonly="readonly" placeholder="<?php echo $pid; ?>">

The value of "$pid" is not null, I already got the value from database. Then I would like to get that value and pass to another php file. So I tried this code :

    <?php
        $ptid=$_POST['pid'];
    ?>

I tried printing this out, but somehow there's no result. Is there anyway to get the value?