PHP - 以3种形式传递变量 - Step1，Step2和Step3 [关闭]

I'm trying to create a form with 3 steps.
1. Filling in the information.
2. Confirmation of information in 1 step.
3. Send email

Whenever I submits the confirmation form, I will receive an email without contents I filled in in Form 1.

Here are the codes:

index.php

<?php
<form id="entry" action="confirm.php" method="post">
<input type="text" name="school" id="school" />
<input type="submit" value="Next Step">
</form>
?>

confirm.php

<?php
<form id="entry" action="thankyou.php" method="post">
<? echo $_POST["school"] ?>
</form>
?>

thankyou.php

<?php
$subject = "Subject";
mail("YourEmail@gmail.com", $subject, $_POST["school"], "From: Testing" );
?>

Any help with be appreciated!

form 3 is incorrect, should be: mail("YourEmail@gmail.com", $subject, $_POST["school"], "From: Testing" ); $_POST["school"].