Php 如果（isset（$_POST['btn-signup']）做什么？

ifisset$\u POST['btn-signup']行的作用是什么？我正在学习php。我已经完成了一些基础课程，现在我通过示例程序学习。这是登录表单的php部分。它显然工作得很好，但我仍然不太理解$u POST的用法

<?php
session_start();
if(isset($_SESSION['userSession'])!="")
{
 header("Location: home.php");
}
include_once 'dbconnect.php';

if(isset($_POST['btn-signup']))
{
 $uname = $MySQLi_CON->real_escape_string(trim($_POST['user_name']));
 $email = $MySQLi_CON->real_escape_string(trim($_POST['user_email']));
 $upass = $MySQLi_CON->real_escape_string(trim($_POST['password']));

 $new_password = password_hash($upass, PASSWORD_DEFAULT);

 $check_email = $MySQLi_CON->query("SELECT email FROM users WHERE email='$email'");
 $count=$check_email->num_rows;

 if($count==0){


  $query = "INSERT INTO users(username,email,password) VALUES('$uname','$email','$new_password')";


  if($MySQLi_CON->query($query))
  {
   $msg = "<div class='alert alert-success'>
      <span class='glyphicon glyphicon-info-sign'></span> &nbsp; successfully registered !
     </div>";
  }
  else
  {
   $msg = "<div class='alert alert-danger'>
      <span class='glyphicon glyphicon-info-sign'></span> &nbsp; error while registering !
     </div>";
  }
 }
 else{


  $msg = "<div class='alert alert-danger'>
     <span class='glyphicon glyphicon-info-sign'></span> &nbsp; sorry email already taken !
    </div>";

 }

 $MySQLi_CON->close();

通过http“post”方法将表单提交到php url时 ，该表单中的命名值将以各自的名称作为键添加到$\u POST。此行检查php文档是否收到名为“btn signup”的值

源表单可能包含以下内容

<input type="text" name="btn-signup" />

---

如果设置了变量$\u POST['btn-signup']，它会说什么。是的，我读了那一页。但根据我们的计划，btn注册尚未设置。它仍在继续

<input type="text" name="btn-signup" />