$alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
$alphaArray = str_split($alpha);

foreach ($alphaArray as $value)
{
    // $value will be each character.  Do your encryption code here.
}

$someArray = array(
    'A' => 'first letter',
    'B' => 'Banana',
    'C' => 'I like desk.'
);

foreach ($someArray as $key => $val) {
    echo $key . ' = ' . $val . '<br />';
    // here is where you can handle encryption
}