Something Something Boolean Error

<?php

require_once "../config/config.php";

class DevSystem
{

    public static function addStudent($conn, $POST) {
        $stno = $POST["stno"];
        $status = $POST["status"];
        $sYear = $POST["sYear"];
        $session = $POST["session"];
        $lastname = $POST["lastname"];
        $firstname = $POST["firstname"];
        $middlename = $POST["middlename"];
        $birthdate = $POST["birthdate"];
        $sex = $POST["sex"];
        $username = $POST["username"];

        // Password generation
        $password_hash = password_hash($firstname, PASSWORD_BCRYPT);
        $gen_exp = explode("$",$password_hash);
        $gen_split = str_split($gen_exp[3], 6);

        // Replacement of non-alphanumeric characters
        $rand_int1 = rand(0, 9);
        $rep1 = str_replace(".", $rand_int1, $gen_split[3]);
        $rand_int2 = rand(0, 9);
        $rep2 = str_replace("/", $rand_int2, $rep1);

        // Hashing for DB
        $final_hash = password_hash($rep2, PASSWORD_BCRYPT);

        $stmt = $conn->db->prepare("INSERT INTO students (stno, status, sYear, session, lastname, firstname, middlename, birthdate, sex, username, password)
        VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)");

        $stmt->bind_param(
            "sssssssssss",
            $stno,
            $status,
            $sYear,
            $session,
            $lastname,
            $firstname,
            $middlename,
            $birthdate,
            $sex,
            $username,
            $final_hash
        );

        $response = '';
        $response .= $stmt->execute() ? 'Student added. Password: ' . $rep2 : $stmt->error;
        if($stmt->error) {
            return $response;
        }
        $stmt->close();
        return $response;
    }

}