function valueFromGetOrPost($parameter)
{
    $shvalue=NULL;
    if ($_GET[$parameter])
    {
        $shvalue=$_GET[$parameter];
    }
    else if (isset($_POST[$parameter]))
    {
        $shvalue=$_POST[$parameter];
    }
    return $shvalue;
}
say by using filter_input
Basically the code check whether a parameter exist either in GET or POST. And then return the value of the parameter.
I think this must be so common it should be there by some built in function already
                        
Use
$_REQUEST(documentation).So your code will look like: