PHP :editing xml files via php form

This is my xml file and my php code. It modifies current information about student. however, what if I have several students. I can only edit the first one (John Doe) how can I edit my second student information then it also changes in my xml file. I have simplified the code here compare to my previous post as my previous code are not working. thanks

<student>
         <firstname>John</firstname>
        <lasttname>Doe</lasttname>
        <student_id>150334</student_id>
        <gender>female</gender>
        <dob>16-09-94</dob>

        <email>[email protected]</email>
        <mobilenumber>12345678</mobilenumber>
        <address>xxx</address>
        <cohort>cohort name</cohort>
        <programme>Softwaree Engineering</programme>
        <mode>FT</mode>
    </student>
     <student>
        <firstname>firnstmae</firstname>
        <lasttname>hahah</lasttname>
        <student_id>150500</student_id>
        <gender>female</gender>
        <dob>24-03-96</dob>

        <email>[email protected]</email>
        <mobilenumber>57056242</mobilenumber>
        <address>addressname</address>
        <cohort>cohort name</cohort>
        <programme>web developement</programme>
        <mode>FT</mode>
    </student>
</students>

<?php

if(isset($_POST['submit'])) {

 $data=simplexml_load_file('studentdb.xml');

 $data->item->firstname=$_POST['firstname'];

 $data->item->lasttname=$_POST['lastname'];

 $data->item->gender=$_POST['gender'];

  $data->item->dob=$_POST['dob'];

   $data->item->email=$_POST['email'];
    $data->item->mobilenumber=$_POST['mobilenumber'];
     $data->item->address=$_POST['address'];
      $data->item->cohort=$_POST['cohort'];
       $data->item->programme=$_POST['programme'];
        $data->item->mode=$_POST['mode'];


$handle=fopen("studentdb.xml","wb");
fwrite($handle,$data->asXML());
fclose($handle);
}

$data=simplexml_load_file('studentdb.xml');
$fName=$data->item->firstname;
$lName=$data->item->lasttname;
$gender=$data->item->gender;
$dob=$data->item->dob;
$email=$data->item->email;
$mobileNo=$data->item->mobilenumber;
$address=$data->item->address;
$cohort=$data->item->cohort;
$programme=$data->item->programme;
$mode=$data->item->mode;


?>

<?php

echo $fName . " ";
echo $lName . " ";
echo $gender . " ";
echo $dob . " ";
echo $email . " ";
echo $mobileNo . " ";
echo $address . " ";
echo $cohort . " ";
echo $programme . " ";
echo $mode . " ";
?>

<form method="post">



    Firstname<input type="text" name="firstname" value="<?php echo $fName;?>" placeholder="firstname" pattern="[A-Z][a-z]+" title="Must start with capital letters!"required/> </br></br>
    Lastname<input type="text" name="lastname" value="<?php echo $lName;?>"placeholder="lastname" pattern="[A-Z][a-z]+" title="Must start with capital letters!"  required/></br></br>
    gender 
    <input type="radio" name="gender" value="M" value="<?php echo $gender;?>"> Male
    <input type="radio" name="gender" value="F" value="<?php echo $gender;?>"> Female<br><br>
    dob<input type="date" name="dob" placeholder="dob" value="<?php echo $dob;?>" /> </br></br>
    email<input type="text" name="email" placeholder="email" value="<?php echo $email;?>"/></br></br>
    mobile No:<input type="text" name="mobilenumber" value="<?php echo $mobileNo;?>" placeholder="mobileno" pattern="5[0-9][0-9][0-9][0-9][0-9][0-9][0-9]" title="Must start with 5 followed by 7 digits" required/></br></br>
    address<input type="text" name="address" placeholder="address" value="<?php echo $address;?>"  title="Must start with capital letters!"required/></br></br>
    cohort<input type="text" name="cohort" placeholder="cohort" value="<?php echo $cohort;?>" required /></br></br>
    programme<input type="text" name="programme" placeholder="programme" value="<?php echo $programme;?>" required //></br></br>
    mode<input type="radio" name="mode" value="FT" value="<?php echo $mode;?>"> Full-Time
    <input type="radio" name="mode" value="PT"> Part-Time<br>
    <br>


 <input name="submit" type="submit" />



</form>