Access to PHP $_GET and $_POST in wordpress - php

Question

In a wordpress page I have a form where, after the pressure of submit botton, the same page is reloaded and it sends an email with the information in the form. The problem is that, as soon as I try to save the page, if in the php code there is a $_GET[],$_POST[] or $_REQUEST[] array, wordpress does not let me to save the changes, and the last code I wrote (those with $_variables) disappears. If I make an external php script and I call it in the wordpress page, I get "Internal Server Error" when I connect to the page.
I tried with two different php wp plugins, and I have the same problem in both.
Does anyone know why?
EDIT:
Here is an example of code I tried, it is the simpest example I found and it is still not working:
<?php
function test() {
echo $_POST["user"];
}
if (isset($_POST[])){ //If it is the first time, it does nothing
test();
}
?>
Here is the form:
<form action="" method="post">
<input type="text" name="user" placeholder="enter a text" />
<input type="submit" value="submit" onclick="test()" />
</form>
Anyway it is as if the wordpress parser recognizes the variables and refuses to save, if I wrote, for instance, only the following code `
<?php
$_GET[];
?>
it give me the same problem, but writing
<?php
$_HELLO[];
?>
though it returns error when I call the page in the browser (obviously considering that $_HELL does not exist), it let me save.
EDIT 2: Very strange, if I print all the post array with
`<?php
var_dump($_POST);
?>`
it works, and it prints "array(1) { ["user"]=> string(3) "123" }",
BUT IF I TRY TO ACCESS TO THE ELEMENT WITH THE KEY "USER" USING [] IT GIVES ME ERROR!

Answer 1

$_POST and $_GET are also work in wordpress too..
Remove the square bracket[ ] on $_POST, it will work.
<form action="" method="post">
<input type="text" name="user" placeholder="enter a text" />
<input type="submit" value="submit" onclick="test()" />
</form>
<?php
function test() {
echo $_POST["user"];
}
if (isset($_POST)){ //If it is the first time, it does nothing
test();
}
?>
Hope this will help you!!