Radio Button Group Produce a Zero Value When Form Method=POST - php

Question

I have a group of radio buttons placed in a form with method post. But anytime the form is post the value from the radio button is zero. Below is the code
<form method="POST" action='insert_per.php'>
<table >
<caption>
Grant Permissions
</caption>
<thead>
<tr>
<th class="center"><strong>Name</strong></th>
<th class="center"><strong>Email</strong></th>
<?PHP if($_SESSION['user_group'] == 63){
echo '<th class="center"><strong>System Admin</strong></th>';
echo '<th class="center"><strong>Admin</strong></th>';
echo '<th class="center"><strong>User</strong></th>';
} ?>
<th class="center"><strong></strong></th>
</tr>
</thead>
<?php
if($stmt->execute()){
// output data of each row
while($rows = $stmt->fetch(PDO::FETCH_ASSOC)){ ?>
<tr>
<td border="0" type="hidden" style="display:none;"><input type="hidden" name="hidden" value=<?php echo $rows['member_id']; ?></td>
<td class="center"><?php echo $rows['username']; ?></td>
<td class="center"><?php echo $rows['email']; ?></td>
<?PHP if($_SESSION['user_group'] == 63){ ?>
<td class="center"><input type='radio' id=<?php echo $rows['username']; ?> name="gp" value="system admin"
<?php echo ($rows['permission']== 31 )?'checked':'' ?>></input> </td>
<td class="center"><input type='radio' id=<?php echo $rows['username']; ?> name="gp" value="admin"
<?php echo ($rows['permission']== 15 )?'checked':'' ?> ></input> </td>
<td class="center"><input type='radio' id=<?php echo $rows['username']; ?> name="gp" value="user"
<?php echo ($rows['permission']== 1 )?'checked':'' ?> ></input> </td> <?PHP } ?>
<td class="center"><button name="SUBMIT" type="SUBMIT">submit</button></td>
</tr>
<?php
}
}
?>
</table>
</form>
Below is the code for the insert_per.php
echo "this is the gp "+ $_POST['gp'];
echo "this is the gp "+ $_POST['hidden'];
try{
$stmt= $dbh->prepare("UPDATE `members_groups` SET `group_id`=(SELECT group_id FROM groups WHERE
group_name = ? ) WHERE member_id = ?");
$stmt->bindParam(1, $_POST['gp'] , PDO::PARAM_STR);
$stmt->bindParam(2 , $_POST['hidden'] , PDO::PARAM_INT);
if($stmt->execute()) {
echo " the privilege was granted successfully ".'<BR>';
echo "<a href='user_acc.php'>View result</a>";
}
} catch(PDOException $e)
{
echo "An error occured ".$e->getMessage();
}
The radio buttons are expected to produce one of the following values
"system admin"
"admin"
"user"
but it always produces zero "0" value.