#PHP 582

<? list($i,$k,$v)=array_map(function($v){return str_split(preg_replace('#[^A-Z]#','',strtr(strtoupper($v),'J','I')));},$argv);@$i=array_flip;$k=$i($k)+$i(range('A','Z'));unset($k['J']);$k=array_keys($k);$q=$i($k);for($i=1;$i<count($v);$i+=2){if ($v[$i-1]==$v[$i]){array_splice($v,$i,0,'X');}}if(count($v)%2)$v[]='X';for($i=1;$i<count($v);$i+=2){$c=(int)($q[$v[$i-1]]/5);$d=$q[$v[$i-1]]%5;$e=(int)($q[$v[$i]]/5);$f=$q[$v[$i]]%5;if($c==$e){$d=($d+1)%5;$f=($f+1)%5;}elseif($d==$f){$c=($c+1)%5;$e=($e+1)%5;}else{$t=$f;$f=$d;$d=$t;}$v[$i-1]=$k[$c*5+$d];$v[$i]=$k[$e*5+$f];}echo join($v);

Ungolfed
Decoder

outputs

$ php playfair.php "Stack Overflow" "The cat crept into the crypt, crapper, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFECYNRDFMSVTVKBTMMY
$ php playfair.php "This was codegolf?" "The full J answers is shorter than my preparation code :("
HIOKVGFHCMWTKZWSIYWIEPWAMWTCPNXQZKMOMEHSCPODEA

PHP 582

<? list($i,$k,$v)=array_map(function($v){return str_split(preg_replace('#[^A-Z]#','',strtr(strtoupper($v),'J','I')));},$argv);@$i=array_flip;$k=$i($k)+$i(range('A','Z'));unset($k['J']);$k=array_keys($k);$q=$i($k);for($i=1;$i<count($v);$i+=2){if ($v[$i-1]==$v[$i]){array_splice($v,$i,0,'X');}}if(count($v)%2)$v[]='X';for($i=1;$i<count($v);$i+=2){$c=(int)($q[$v[$i-1]]/5);$d=$q[$v[$i-1]]%5;$e=(int)($q[$v[$i]]/5);$f=$q[$v[$i]]%5;if($c==$e){$d=($d+1)%5;$f=($f+1)%5;}elseif($d==$f){$c=($c+1)%5;$e=($e+1)%5;}else{$t=$f;$f=$d;$d=$t;}$v[$i-1]=$k[$c*5+$d];$v[$i]=$k[$e*5+$f];}echo join($v);

Ungolfed
Decoder

outputs

$ php playfair.php "Stack Overflow" "The cat crept into the crypt, crapper, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFECYNRDFMSVTVKBTMMY
$ php playfair.php "This was codegolf?" "The full J answers is shorter than my preparation code :("
HIOKVGFHCMWTKZWSIYWIEPWAMWTCPNXQZKMOMEHSCPODEA

#PHP 582

<? list($i,$k,$v)=array_map(function($v){return str_split(preg_replace('#[^A-Z]#','',strtr(strtoupper($v),'J','I')));},$argv);@$i=array_flip;$k=$i($k)+$i(range('A','Z'));unset($k['J']);$k=array_keys($k);$q=$i($k);for($i=1;$i<count($v);$i+=2){if ($v[$i-1]==$v[$i]){array_splice($v,$i,0,'X');}}if(count($v)%2)$v[]='X';for($i=1;$i<count($v);$i+=2){$c=(int)($q[$v[$i-1]]/5);$d=$q[$v[$i-1]]%5;$e=(int)($q[$v[$i]]/5);$f=$q[$v[$i]]%5;if($c==$e){$d=($d+1)%5;$f=($f+1)%5;}elseif($d==$f){$c=($c+1)%5;$e=($e+1)%5;}else{$t=$f;$f=$d;$d=$t;}$v[$i-1]=$k[$c*5+$d];$v[$i]=$k[$e*5+$f];}echo join($v);

Ungolfed

outputs

$ php playfair.php "Stack Overflow" "The cat crept into the crypt, crapper, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFECYNRDFMSVTVKBTMMY
$ php playfair.php "This was codegolf?" "The full J answers is shorter than my preparation code :("
HIOKVGFHCMWTKZWSIYWIEPWAMWTCPNXQZKMOMEHSCPODEA

#PHP 582

<? list($i,$k,$v)=array_map(function($v){return str_split(preg_replace('#[^A-Z]#','',strtr(strtoupper($v),'J','I')));},$argv);@$i=array_flip;$k=$i($k)+$i(range('A','Z'));unset($k['J']);$k=array_keys($k);$q=$i($k);for($i=1;$i<count($v);$i+=2){if ($v[$i-1]==$v[$i]){array_splice($v,$i,0,'X');}}if(count($v)%2)$v[]='X';for($i=1;$i<count($v);$i+=2){$c=(int)($q[$v[$i-1]]/5);$d=$q[$v[$i-1]]%5;$e=(int)($q[$v[$i]]/5);$f=$q[$v[$i]]%5;if($c==$e){$d=($d+1)%5;$f=($f+1)%5;}elseif($d==$f){$c=($c+1)%5;$e=($e+1)%5;}else{$t=$f;$f=$d;$d=$t;}$v[$i-1]=$k[$c*5+$d];$v[$i]=$k[$e*5+$f];}echo join($v);

Ungolfed
Decoder

outputs

$ php playfair.php "Stack Overflow" "The cat crept into the crypt, crapper, and crept out again."
SIRACARDFMVUICVSMORDZNAKECMZMFECYNRDFMSVTVKBTMMY
$ php playfair.php "This was codegolf?" "The full J answers is shorter than my preparation code :("
HIOKVGFHCMWTKZWSIYWIEPWAMWTCPNXQZKMOMEHSCPODEA