Increase/decrease parameter in query parameter $_GET through links - php

Question

Recently, I started to learn PHP through some course I found on my college website (don't worry, this is not homework, I'm doing this on my own)
and I'm stuck. The assignment goes like this:
Create a page with two links, one for increasing and one for decreasing parameter 'n' which should be accessed through $_GET query parameter. If 'n' is not set, assume it's value is 4.
So, I tried something like this (just for increasing at first):
<form action="<?php $_PHP_SELF ?>" method="GET">
<input type="hidden" name="n" value="4">
<input type="submit">
</form>
and my PHP code goes like this:
<?php
$var = 4;
if(!isset($_GET['n'])) {
$_GET['n'] = $var;
} else {
$var = $_GET['n'];
$var++;
$_GET['n'] = $var;
}
echo $_GET['n'];
?>
But this does not seem to work, at all. I'm guessing the 'n' should automatically change in the URL too. Also, how can I have to "submit" buttons, one for increase, one for decrease?
Can anyone help (with some good instructive explanation if possible) and if it's possible to make it with just HTML links because the course didn't go through forms yet.

Answer 1

in the form code:
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="GET">
<input type="hidden" name="n" value="<?php echo $var; ?>">
<input type="submit">
</form>
EDIT
to make it increase / decrease:
<?php
if(!isset($_GET['n'])) {
$var = 4;
} else {
$var = $_GET['n'];
}
echo 'Value = ' . $var;
?>
<p>
Increase
Decrease
</p>

Answer 2

Your PHP code could be simplified to the following:
if (!isset($_GET['n'])) {
$_GET['n'] = 4;
}
$_GET['n']++; // you can do that
However, you probably won't want to increment or decrement $_GET['n'] directly. Try keeping just the if part, then incrementing or decrementing when you output the link:
<?php
if (!isset($_GET['n'])) {
$_GET['n'] = 4;
}
?>
increment n
decrement n
Note that don't need a form to pass parameters through $_GET or $_POST — you can just build a URL that passes a parameter, like I did here.