Replace Content on Page Based on URL Parameter with PHP - php

Question

I'd like to replace content within my page based on the URL parameter.
Ideally I'd like to use PHP to get:
if {{parameter is X}} display {{content X}}
if {{parameter is Y}} display {{content Y}}
..for a few pages.
Current set up:
<?php if ($CURRENT_PAGE == "Index") { ?>
<div id="firstDiv">this is the standard page</div>
<?php } ?>
<?php if ($CURRENT_PAGE == "p1") { ?>
<div id-"secondDiv">this is a variation of the page</div>
<?php } ?>
And using include("includes/content.php"); to call the html blocks to the page
The firstDiv displays in index.php as expected, but adding the URL parameter changes nothing - the same div still shows (I'd like it to be replaced with the secondDiv)
It seems $CURRENT_PAGE doesn't like URL parameters - what is the alternative?
Hopefully this makes sense, I'm pretty new to PHP. Happy to provide more details if required.
Thanks in advance for any help.
-- UPDATE --
Thank you for the answers so far!
It seems I missed part of my own code (Thanks to vivek_23 for making me realise this - I'm using a template, excuse me!!)
I have a config file that defines which page is which, as so:
<?php
switch ($_SERVER["SCRIPT_NAME"]) {
case "index.php/?p=1":
$CURRENT_PAGE = "p1";
break;
default:
$CURRENT_PAGE = "Index";
}
?>
Before I learn $_GET, is there a way I can use my current set up?
Thanks again.
-- UPDATE 2 --
I have switched to using the $_GET method, which seems to be working well so far. My issue now is when the parameter is not set it is giving an undefined error. I'll try to remember to update with the fix.
$p = ($_GET['i']);
if($p == "1"){
echo '<div id="firstDiv"><p>this is the first div</p></div>';
}
Thanks to the two answerers below who suggested using $_GET

Answer 1

You can used $_GET like
if($_GET['p']==1){
echo '<div id="firstDiv">this is the standard page</div>';
}else if($_GET['p']==2){
echo '<div id="secondDiv">this is a variation of the page</div>';
}
The other way! you can used basename() with $_SERVER['PHP_SELF']
//echo basename($_SERVER['PHP_SELF']); first execute this and check the result
if(basename($_SERVER['PHP_SELF']) == 'index'){
echo '<div id="firstDiv">this is the standard page</div>';
}else{
echo '<div id="secondDiv">this is a variation of the page</div>';
}

Answer 2

You need to send the parameters on the URL query string, like:
yourdomain.com?p=1
So, with this URL, the query string is "?p=1", where you have a GET parameter named 'p' with a value of '1'.
In PHP to read a GET parameter you can use the associative array $_GET, like this:
$current_page = $_GET['p'];
echo $current_page; // returns '1'
The rest of your logic is OK, you can display one div or the other based on the value of the p parameter.
You can read more about how to read query string parameters here: http://php.net/manual/en/reserved.variables.get.php