Add a set of querystring params to a URL

• Your code is very spaced out. It doesn't need to be.
• Consider using industry-standard style by using Egyptian braces for control structures like if and foreach.
• Get rid of the $http_host variable; you never use it.
• Consider generalising your function to accept any URL instead of pulling REQUEST_URI.
• You're kind of reinventing the wheel; PHP has the built-in functions parse_str and http_build_query that you can use.
• Do you intend to replace existing variables, or just to append them to the request?

Here's an example that takes advantage of the built-in functions, and intelligently merges in conflicts. (ideone)

/**
 *  Like array_merge, but will recursively merge array values.
 *
 *  @param array $a1
 *      The array to be merged into.
 *  @param array $a2
 *      The array to merge in. Overwrites $a1, when string keys conflict.
 *      Numeric keys will just be appended.
 *  @return array
 *      The array, post-merge.
 */
function merge_query_var_arrays($a1, $a2) {
    foreach ($a2 as $k2 => $v2)
        if (is_string($k2))
            $a1[$k2] = isset($a1[$k2]) && is_array($v2) ? merge_query_var_arrays($a1[$k2], $v2) : $v2;
        else
            $a1[] = $v2;
    return $a1;
}

/**
 *  @param string $query_string
 *      The URL or query string to add to.
 *  @param string|array $vars_to_add
 *      Either a string in var=val&[...] format, or an array.
 *  @return string
 *      The new query string. Duplicate vars are overwritten.
 */
function add_query_vars($query_string, $vars_to_add) {
    if (is_string($vars_to_add))
        parse_str($vars_to_add, $vars_to_add);
    if (preg_match('/.*\?/', $query_string, $match))
        $query_string = preg_replace('/.*\?/', '', $query_string);
    parse_str($query_string, $query_vars);

    $query_vars = merge_query_var_arrays($query_vars, $vars_to_add);
    return @$match[0] . http_build_query($query_vars);
}

I use this function in my pagination script to append query strings to the ?p= (page number) query, this enables me to sort a table from the same page or set the Limit in a table populated by MySQL

    public function modQuery($add_to, $rem_from = array(), $clear_all = false){
        if ($clear_all){
            $query_string = array();
        }else{
            parse_str($_SERVER['QUERY_STRING'], $query_string);
        }
        if (!is_array($add_to)){ $add_to = array(); }
            $query_string = array_merge($query_string, $add_to);
        if (!is_array($rem_from)){ $rem_from = array($rem_from); }
            foreach($rem_from as $key){
        unset($query_string[$key]);
        }
    return http_build_query($query_string);
    }

Usage:

You can use it in a href:

'<a href="?'.modQuery(array('query'=>'queryValue','query'=>'queryValue'))).'">Some Query</a>'