How to add number to username if it already exists

I am trying to make a school portal system for my coursework. Every student has a username which consists of the first letter of their name, and first 4 letters of their surname. To create this username automatically, I am using this in the page where the admins adds students.

if (isset($_POST["btnAddUser"])) {
    //preparing statement to protect against sql injections
    $stmt = $conn->prepare("INSERT INTO tbluser (Username, Password, Role) VALUES (?,?,?)");
    $stmt->bind_param("sss", $usernamenew3, $password, $a);
    $password = $_POST["s_password"];
    $password = md5($password);
    $name = $_POST["s_name"];
    $surname = $_POST["s_surname"];
    $dob = $_POST["s_dob"];
    $a = "Admin";
    $usernamenew = substr($name, 0, 1);
    $usernamenew1 = substr($surname, 0, 4);
    $usernamenew3 = $usernamenew.$usernamenew1;

However, if for example two students with the same surname, and same initial letter of their name are entered, it would come up with an error, so I need it to add 01 the first time that username is used, 02 the second time and so on. Example. Name = Test, Surname = Student For this example I would like the first username with those letters to be TStud01, the second to be TStud02...

Solution

You need to fetch the count of usernames starting with the common pattern from the database and then increment this by 1. Then you can pad it with 0 and save that in the database.

Few points to note:

Never store passwords in clear text or using MD5/SHA1! Only store password hashes created using PHP's password_hash(), which you can then verify using password_verify(). Take a look at this post: How to use password_hash and learn more about bcrypt & password hashing in PHP. Make sure the column is VARCHAR(255).
Use mb_substr() instead, because names can contain non-latin letters.
To use LIKE with mysqli you must concatenate the % in PHP and then bind that variable to the query.
To understand how to get the count of matching rows with MySQLi see How to get count of rows in MySQL table using PHP?

if (isset($_POST["btnAddUser"])) {
    $password = password_hash($_POST["s_password"], PASSWORD_DEFAULT);
    $name = $_POST["s_name"];
    $surname = $_POST["s_surname"];
    $dob = $_POST["s_dob"];
    $a = "Admin";
    $fistLetter = mb_substr($name, 0, 1);
    $shortSurname = mb_substr($surname, 0, 4);
    $usernamenew = $fistLetter.$shortSurname;

    $searchString = $usernamenew.'%';

    $stmt = $conn->prepare('SELECT COUNT(Username) FROM tbluser WHERE Username LIKE ?');
    $stmt->bind_param('s', $searchString);
    $stmt->execute();
    $countUsername = $stmt->get_result()->fetch_row()[0];

    $usernamenew .= str_pad($countUsername+1, 2, '0', STR_PAD_LEFT);

    //preparing statement to protect against sql injections
    $stmt = $conn->prepare("INSERT INTO tbluser (Username, Password, Role) VALUES (?,?,?)");
    $stmt->bind_param("sss", $usernamenew, $password, $a);
    $stmt->execute();
}

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