It's actually much easier to do this in the server in PHP code, anyway.
You just check to see if you got any values in $_GET["keyword"] and, if so, just convert all the values into an IN( ) list for your SQL query.
I don't use PHP, but I *believe* it would be something like this:
Code:
if ( isset($_GET["keyword"]) )
{
$list = "";
for ( $i = 0; $i < count( $_GET["keyword"] ); ++$i )
{
if ( $i > 0 ) $list .= ",";
$list .= "'" + mysql_real_esape_string($_GET["keyword"][$i]) + "'";
}
$sql = "SELECT * FROM table WHERE somefield IN (" . $list . ")";
...
}