• wasEncryptVIGENERE takes as parameter a plain-text and an encryption key and returns cypher-text.
• wasDecryptVIGENERE takes as parameter a cypher-text and an encryption key and returns plain-text.

VIGENERE.php

#!/usr/bin/php -e
<?php
 
///////////////////////////////////////////////////////////////////////////
//  Copyright (C) Wizardry and Steamworks 2015 - License: GNU GPLv3      //
///////////////////////////////////////////////////////////////////////////
function wasReversePermuteArrayElements($input, $times) {
    if ($times == 0) return $input;
    return wasReversePermuteArrayElements(
        array_merge(
            array_slice(
                $input,
                1
            ),
            array_slice(
                $input,
                0,
                1
            )
        ),
    --$times
    );
}
 
///////////////////////////////////////////////////////////////////////////
//  Copyright (C) Wizardry and Steamworks 2015 - License: GNU GPLv3      //
///////////////////////////////////////////////////////////////////////////
function wasVIGENEREExpandKey($input, $enc_key) {
    $exp_key = "";
    $i = 0;
    $j = 0;
    do {
        $p = substr($input, $i, 1);
        if (!ctype_alpha($p)) {
            $exp_key .= $p;
            ++$i;
            continue;
        }
        $m = $j % strlen($enc_key);
        $exp_key .= substr($enc_key, $m, 1);
        ++$j;
        ++$i;
    } while ($i < strlen($input));
    return $exp_key;
}
 
///////////////////////////////////////////////////////////////////////////
//  Copyright (C) Wizardry and Steamworks 2015 - License: GNU GPLv3      //
///////////////////////////////////////////////////////////////////////////
function wasEncryptVIGENERE($input, $enc_key) {
    $a = array
         (
        "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
        "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
         );
 
    // expand the key
    $enc_key = wasVIGENEREExpandKey($input, $enc_key);
 
    $result = "";
    $i = 0;
    do {
        $p = substr($input, $i, 1);
        if (!ctype_alpha($p)) {
            $result .= $p;
            ++$i;
            continue;
        }
        $q = wasReversePermuteArrayElements(
            $a,
            array_search(
                substr(
                    $enc_key,
                    $i,
                    1
                    ),
                $a
            )
        )[
            array_search(
                strtolower($p),
                $a
            )
        ];
        if (ctype_upper($p)) {
            $q = strtoupper($q);
        }
        $result .= $q;
        ++$i;
    } while ($i < strlen($input));
 
    return $result;
}
 
///////////////////////////////////////////////////////////////////////////
//  Copyright (C) Wizardry and Steamworks 2015 - License: GNU GPLv3      //
///////////////////////////////////////////////////////////////////////////
function wasDecryptVIGENERE($input, $enc_key) {
    $a =
    [
        "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
        "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"
    ];
 
    // expand the key
    $enc_key = wasVIGENEREExpandKey($input, $enc_key);
 
    $result = "";
    $i = 0;
    do {
        $p = substr($input, $i, 1);
        if (!ctype_alpha($p)) {
            $result .= $p;
            ++$i;
            continue;
        }
        $q = $a[
            array_search(
                strtolower($p),
                wasReversePermuteArrayElements(
                    $a,
                    array_search(
                        substr(
                            $enc_key,
                            $i,
                            1
                        ),
                    $a
                    )
                )
            )
        ];
        if (ctype_upper($p)) {
            $q = strtoupper($q);
        }
        $result .= $q;
        ++$i;
    } while ($i < strlen($input));
 
    return $result;
}
 
?>

A consistency test call that first encrypts the string It is a good day! and then decrypts the string would be:

// Encrypt the text "It is a good day!!" using the key "vig" and decrypt result
// using the same key.
print wasDecryptVIGENERE(
    wasEncryptVIGENERE(
        "It is a good day!!", 
        "vig"
    ), 
    "vig"
);
 
print "\n";

which should return It is a good day!.