Costruzione matrice di immagini

[starcraftworld]

$string="RBTNBFBBFQBFKBTBBFNBFRBTPBFPBFPBFPBFPBFPBFPBFPBFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFEFFPWFPWFPWFPWFPWFPWFPWFPWFRWTNWFBWFQWFKWTBWFNWFRWTW";
print "
";
        print "<table border='1'>";
        for ($i=0;$i<8;$i++) {
            for ($j=0;$j<8;$j++)
                print substr($string,3*($i*8+$j),3)." - ";
            print "
";
        } //for
        for ($i=0;$i<8;$i++) {
            print "<tr>";
            for ($j=0;$j<8;$j++) {
                $piece=substr($string,3*($i*8+$j),3);
                switch ($piece[0]) {
                    case 'R': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/rw.png[/img]</td>";
                        else
                            print "<td>[img]img/rb.png[/img]</td>";
                    } //case                
                    case 'N': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/nw.png[/img]</td>";
                        else
                            print "<td>[img]img/nb.png[/img]</td>";
                    } //case
                    case 'B': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/bw.png[/img]</td>";
                        else
                            print "<td>[img]img/bb.png[/img]</td>";
                    } //case
                    case 'Q': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/qw.png[/img]</td>";
                        else
                            print "<td>[img]img/qb.png[/img]</td>";
                    } //case
                    case 'K': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/kw.png[/img]</td>";
                        else
                            print "<td>[img]img/kb.png[/img]</td>";
                    } //case
                    case 'P': {
                        if ($piece[1]=="W")
                            print "<td>[img]img/pw.png[/img]</td>";
                        else
                            print "<td>[img]img/pb.png[/img]</td>";
                    } //case
                    case 'E': {
                        print "<td>[img]img/e.png[/img]</td>";
                    } //case
                    default:
                        print "<td>?</td>";
                } //switch
            } //for
            print "</tr>";            
        } //for
        print "</table>";
        print "
"; 

Le prime 5 immagini sono caricate correttamente, poi viene caricata la pb.png al posto della bb.png e poi sono tutte sbagliate, perchè?