base64.php

<?php
$code = '';

$code = preg_replace('/^<\?/', '', $code);
$specialChars = implode('', array_unique(str_split(preg_replace('#[a-z0-9+/]+#i', '', $code))));

file_put_contents('php://stderr', "Special Characters: " . var_export($specialChars, true) . "\n");
$unusedChars = implode('', array_diff(
    str_split('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789/+'),
    array_unique(array_diff(str_split($code), str_split($specialChars)))
));
$unusedChars = substr($unusedChars, 0, strlen($specialChars));

file_put_contents('php://stderr', "Unused Characters: " . var_export($unusedChars, true) . "\n");

if (strlen($specialChars) > strlen($unusedChars)) {
    die("not enough unused chars!\n");
}

$escapedCode = strtr($code, $specialChars, $unusedChars);
$encodedCode = base64_decode($escapedCode);
$iterations = 0;
while (substr(strtr(base64_encode($encodedCode), $unusedChars, $specialChars), -1) !== ';') {
    $code .= ';';
    $escapedCode = strtr($code, $specialChars, $unusedChars);
    $encodedCode = base64_decode($escapedCode);
    $iterations++;
}

file_put_contents('php://stderr', "Padding: {$iterations}\n");

$encodedCode = str_replace("'", "\\'", $encodedCode);

$codeTemplate = "<?eval(strtr(base64_encode('%s'),%s,%s));";

printf($codeTemplate, $encodedCode, $unusedChars, '~' . ~$specialChars);