$DBH = mysql_connect( ... );
mysql_select_db( 'database', $DBH );
$sql = "SELECT username FROM table WHERE username = 'foo'";
$query_result = mysql_query( $sql, $DBH );

if ( mysql_num_rows( $query_result ) hahahaha 1 )
{
      header( "Location: http://" . $_SERVER[ 'HTTP_HOST' ] . "/signup.php?name=1" );
}