GET method appeared in $_SERVER before submit - php

Question

<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" onsubmit="return checkValid(this)" method="GET">
<input type="text" name="Symbol">
<input type="submit" name="submit" value="Search">
</form>
<?php
if($_SERVER["REQUEST_METHOD"] == "GET"){
echo "test";
$CSymbol = $_GET["Symbol"];}
?>
when the page was first load and did not press submit button, the php was executed and "test" was print on the screen, also with error:
Notice: Undefined index: Symbol in test.php on line 179
but if i change everything to POST, problem solved. Why is that, what's different btn GET and POST?

Answer 1

GET values are sent in the URL, POST in the HTTP Body. So "GETting" values can alsways be done but may be empty. POST has to be manually created. If you want to use GET in this case however. Guard it:
<?php
if($_SERVER["REQUEST_METHOD"] == "GET" && isset($_GET["Symbol"]){
echo "test";
$CSymbol = $_GET["Symbol"];}
?>
Reference Link

Answer 2

GET and POST are two different types of HTTP requests.
According to WikiPedia:
GET requests a representation of the specified resource. Note that GET should
not be used for operations that cause side-effects, such as using it for taking
actions in web applications. One reason for this is that GET may be used
arbitrarily by robots or crawlers, which should not need to consider the side
effects that a request should cause.
and
POST submits data to be processed (e.g., from an HTML form) to the identified
resource. The data is included in the body of the request. This may result in
the creation of a new resource or the updates of existing resources or both.
So essentially GET is used to retrieve remote data, and POST is used to insert/update remote data.
Authors of services which use the HTTP protocol SHOULD NOT use GET based forms
for the submission of sensitive data, because this will cause this data to be
encoded in the Request-URI. Many existing servers, proxies, and user agents will
log the request URI in some place where it might be visible to third parties.
Servers can use POST-based form submission instead
Finally, an important consideration when using GET for AJAX requests is that some browsers - IE in particular - will cache the results of a GET request. So if you, for example, poll using the same GET request you will always get back the same results, even if the data you are querying is being updated server-side. One way to alleviate this problem is to make the URL unique for each request by appending a timestamp.
For your above code..
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" onsubmit="return checkValid(this)" method="GET">
<input type="text" name="Symbol">
<input type="submit" name="submit" value="Search">
</form>
<?php
if($_SERVER["REQUEST_METHOD"] == "GET" && !empty($_GET['Symbol'])){
echo "test";
$CSymbol = $_GET["Symbol"];}
?>

Answer 3

Every http request that is not a POST counts as a GET request, and so every page that loads up has an empty GET request, php will create a super global $_GET array with every non post request.
Hope this helps you understand what is going on.