My script doesn't work as expected - php

Question

This is the button
<a href="index.php?p=contact">contact<a>
This is the php script:
<?php
$p = isset($_GET['p']);
if($p == "artist")
{
include 'artist.php';
}
if($p == "contact")
{
include 'contact.php';
}
if($p == "releases")
{
include 'releases.php';
}
if($p == "downloads")
{
include 'downloads.php';
}
else
{
include 'home.php';
}
?>
So my script should include contact.php when I hit the button, but instead of including only contact.php it includes all php files. (this happens also with the other buttons).

Answer 1

Right now your $p variable equals true (this is what isset returns).
Change $p = isset($_GET['p']); to $p = $_GET['p']; and you'll be good
Even better:
$p = isset($_GET['p']) ? $_GET['p'] : false;
in this case you're secured against p being null
EDIT:
There is also another issue with your code - last else statement. It is always true when $p is different than downloads. So either you change every if to else if like this:
if($p == 'artist')
{
include 'artist.php';
}
else if($p == 'contact')
{
include 'contact.php';
}
(...)
else
{
include 'home.php';
}
or change this to switch statement:
switch($p)
{
case 'artist':
include 'artist.php';
break;
(...)
default:
include 'home.php';
}

Answer 2

isset($_GET['p']) returns true or false, so the code comparing $p to some strings will always return true and run the code inside the if block.
Change $p = isset($_GET['p']) simply to $p = $_GET['p']

Answer 3

To make this (more) reusable, you should alter your script a bit. Create a simple whitelist, check it the parameter is allowed, and if yes, include the $_GET['p']'s value directly:
$allowed = false;
if (isset($_GET['p']))
{
switch $_GET['p'] {
case 'contact':
$allowed = true;
break;
case 'artist':
$allowed = true;
break;
// and so on for all your IFs
}
if ($allowed === true) {
include $_GET['p'].'.php'
}
else
{
die('illegal parameter found')
}
}