<?php
// This would output '/client/?s=word&foo=bar'
echo add_query_arg('foo', 'bar');
// This would output '/client/?s=word&foo=bar&baz=tiny'
$arr_params= array ('foo'=> 'bar',
'baz'=> 'tiny');
echo add_query_arg($arr_params);
?>
<?php
// This would output 'http://blog.example.com/2009/04/16/?hello=world'
echo add_query_arg('hello', 'world', 'http://blog.example.com/2009/04/16/');
?>
http://codex.wordpress.org/Function_Reference/add_query_arg(add_query_arg)