Passed data value will stay on page after submitting form - php

Question

I have a 2 pages and I want to stay the data value that i passed from page1.php
page1.php
<form action='page2.php' method='POST' class='form-inline'>
<input type='submit' class='btn btn-info' value='Manage Child'
name='manage'>;
<input type='hidden' value='<?php echo $id;?>' name='id'>
</form>
page2.php
$id = $_POST['id'];
if(isset($_POST['passinfo'])){
$studentid=$_POST['studid'];
}else{
$studentid='';
}
<form>
<input type='hidden' name='parentid' value='<?php echo $id;?>'
<input type='hidden' name='studentid' value='<?php echo $studentid;?>'
<input type='text' name='studentname'>
</form>
<form action='' method='POST'>
<input type='hidden' name=studid value='<?php echo $stud_id?'>
<input type='submit' class='btn btn-primary' value='Assign' name='passinfo'>
</form>
my problem is if I click the 'passinfo' button and submit the 'studid' value the '$parentid' value will be unindentified variable. My question is how do I maintain the value of the '$parentid' even if I submit a form

Answer 1

You must place hidden inputs in one form:
<form action='' method='POST'>
<input type="hidden" name="parentid" value='<?=$id?>'>
<input type="hidden" name="studentid" value='<?=$studentid?>'>
<input type='hidden' name=studid value='<?=$stud_id?'>
<input type='submit' class='btn btn-primary' value='Assign' name='passinfo'>
</form>