php4 to php5

I suppose I am behind the time in migrating from php4 to 5.  That said, 
my main problem is getting variables recognised.  I can see how to do it 
- you need to identify all the variables that could be sent from the 
page and then assign a name, as in $myvar=$_GET['myvar'] and ditto for 
POST variables if any.  Is there a function to convert all get and post 
variables to $_GET and $_POST, or is that a security concern?  The 
further I have got is this:

function php5($var) {
    if(ISSET($_POST[$var])&&$_POST[$var]<>"") {
        $newvar=$_POST[$var];
    } else {
        $newvar=$_GET[$var];
    }
    return $newvar;
}

$input_variable=php5('input_variable');

however I would like to do this for all variable, I tried iterating 
through the $_GET and $_POST arrays but could not get it to work.

Anyway the main reason I am writing is:

- where the web page form uses the POST method, but there are parameters 
in the URL after the ? sign, these 'get' parameters seem to stay there 
when you submit the form from eg <input type=submit> button.  So, let's 
say the script will add a line if addlines=y, if before you submit the 
form you have ?addline=y in the URL you will continue to add lines 
according to the script even though there is no hidden variable on the 
page called 'addline', because every time you submit the form with POST 
you have addline=y in the URL - if the script looks at GET variables 
then this will feed into the script.  This does not seem to happen with 
php4 - if the page does not submit an 'addline' variable from a form 
field, it will not feed into the script.  So what's the rationale for 
that (the URL submitting the variables) and what is the usual solution?  
The problem arises on this particular page because of a mix of buttons, 
some are javascript and send the addline=y with onClick.

John