add character to my field in form periodic in a loop - php

Question

i need a php code that user click on send button in my form automatically add a different character like a - b - c -d to field input.
for example this is my form html code.
<form action="action.php" method="post">
<input type="text" id="phone" name="phone">
<input type="submit" value="send">
</form>
and in action.php like this
echo $_POST['phone'];
user enter 002255 in phone number field and click send button
print 002255 now .
but i want when user click send button for first time , print a002255
and when click send button for second time , print b002255
and next c002255 and next d002255
then make a loop and when click for fifth time , print a002255 again.

Answer 1

What will happened after reaching letter Z? In example below it will reset. I think you need to create an array and index to it. So when user click button index goes +1. Index will be on session because it has to store number. Am beginner too and trying to figure it out. Example:
session_start();
$alphabet = array('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z');
if(isset($_POST["submit"])) {
if($_SESSION["Number"] == 0) {
print_r($alphabet[0] . "002255");
$_SESSION["Number"] += 1;
} else {
print_r($alphabet[$_SESSION["Number"]] . "002255");
$_SESSION["Number"] += 1;
if($_SESSION["Number"] > 26) {
$_SESSION["Number"] = 0;
}
}
}