PHP: Change a piece of the URL

changepiece.php

function changepiece($parameter, $value) 
{ 
    $params = array(); 
    $output = "?"; 
    $firstRun = true; 
    foreach($_GET as $key=>$val) 
    { 
        if($key != $parameter) 
        { 
            if(!$firstRun) 
            { 
                $output .= "&"; 
            } 
        else 
        { 
            $firstRun = false; 
        } 
        $output .= $key."=".urlencode($val); 
        } 
    } 
    if(!$firstRun) 
    $output .= "&"; 
    $output .= $parameter."=".urlencode($value); 
    return htmlentities($output); 
}