函数和确认密码中未定义的索引不起作用

我正在将表单验证代码转换为函数，但问题是它给出了错误$confirm变量的未定义索引，该变量已经定义并确认密码不起作用。

功能

function formValidation($action,$confirm){
    $result = "";
    $input = $_POST[$action];
    $confirm = $_POST[$confirm];
    // For Email Validation
    $find = 'email';
    $path = $action;
    $pos = strpos($path,$find);
    if(empty(user_input($input))){
        $result = "$action is missing";
    }elseif($pos !== false){
        $email = filter_var($input, FILTER_SANITIZE_EMAIL);
        if(!filter_var($email, FILTER_VALIDATE_EMAIL)){
            $result = "Email invalid format";
        }
    }elseif($confirm !== $_POST['password']){
        $result = "password is not confirm";
    }
    return $result;
}

有一种方法，我一次调用函数，它检查所有字段并返回错误

触发

$email_err = $password_err = $username_err = $confirmPWD_err = "";
if(isset($_POST['submit'])){
    $email_err = formValidation('prd_email','');
    $password_err = formValidation('password','');
    $username_err = formValidation('username','');
    $confirmPWD_err = formValidation('password','confirm');
}

HTML

<form method="post">
    <div class="form-group">
        <input class="form-control" placeholder="username" name="username" type="text" />
            <?php echo $username_err ?>
    </div>
    <div class="form-group">
        <input class="form-control" placeholder="email" name="prd_email" type="text" />
        <?php echo $email_err ?>
    </div>
    <div class="form-group">
        <input class="form-control" placeholder="password" name="password" type="password" />
        <?php echo $password_err ?>
    </div>
    <div class="form-group">
        <input class="form-control" placeholder="Confirm Password" name="confirm" type="password" />
        <?php echo $confirmPWD_err ?>
    </div>
    <input type="submit" class="btn btn-success" name="submit" value="submit" />
</form>