Understanding GET in PHP, Undefined index:name Errors at POST

Thank you for your cooperation.

I wanted to generate a form tag in PHP and reflect the data I sent in input and submit on the same page, but it didn't work.

The following is the situation.

test.phpfile

<form name="name" action="test.php" method="GET">
    <Label for="name">Name:</Label>
    <input name="name" type="text">
    <input type="submit" value="send">
</form>

<pre>
<?php

echohtmlspecialchars($_GET["name"]);

?>
</pre>

*Tags such as html and body are not included.

On the same page, if you write any name in the name field and press Send, you will receive the Undefined index:name error.

When I looked into it, it seemed that the error occurred because ($_GET["name"]); was undefined when I first accessed test.php, and
As a countermeasure,

if(!empty($_POST["submit"])){}

Or

if(isset($_POST["submit"])){}

Use to verify definitions

There was an article saying that, but what does the definition of $_GET["name"] refer to in the text?

In the tag of the information sent from the form, I wondered if I could get the "name" with the GET variable defined by name="name".
Do I need to define it separately?

I'm sorry to trouble you, but I'd appreciate it if you could give me some advice.