html - how to get form POST value in php

I need how to get form POST value on the same page. Below is my script to try and extract the value as well as the form html:

    

        
            
            Boy's
            Girl's
        
    
 

My Php code:

if (isset($_POST)) {  
    $pg_type =$_POST['lamp'];
    echo $pg_type;die;
}

I have echo the my variabe it is showing like:

Notice: Undefined index: lamp in C:\xampp\www\htdocs\rentozy\assets\includes\list-header.php on line 20