Subtraction of variable PHP - php

Question

i cant figure out how can i subtract 2 from a variable ($energia) each time i click on a html link. Problem is that it only subtracts once, so the output will always be 18.
<?php
$energia = 20;
if(isset($_GET["action"]) && $energia != 0)
{
$energia -=2;
}
}
?>
<a href="?action" >subtract</a> <?php echo $energia2; ?>

Answer 1

Does this give you the result you want?
<?php
//checks if num is set, if not, set it to 20 by default
/*
The following line is the same as
if(isset($_GET["num"]))
{
$energia = $_GET["num"];
}
else
{
$energia = 20;
}
*/
//value condition if true else
$energia = isset($_GET["num"]) ? $_GET["num"] : 20;
//if action was clicked and energia is not 0 (not to go in negatives)
//remove && $energia != 0 to allow negative numbers
if(isset($_GET["action"]) && $energia != 0)
{
//substract 2 from energia
$energia -=2;
}
?>
This line sets action to true AND also stores the value of energia in the substact link string. When clicked, it is retrieved with $_GET["num"]. After the link, the current value (same as in the link) is shown to the user.
<a href="?action=true&num=<?php echo $energia; ?>" >subtract</a> <?php echo $energia; ?>

Answer 2

here is a jquery solution: jsfiddle
html
<span class="energia"></span>
<br/>
subtract
jquery
var energia = 20;
$('.energia').text(energia);
$(document).on('click','.subtract', function(){
energia -= 2;
$('.energia').text(energia);
return false;
});