php: converting number to alphabet and vice versa

I don't understand at all the logic you're trying to use in that function. What you're trying to do seems very strange (why does 'a' map to zero and yet 'aa' maps to 26?), but this appears to work. (You will want to use some more test cases, I only checked that it gives the correct output for the case 'aba'.)

function toNum($data) {
    $alphabet = 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'
                       );
    $alpha_flip = array_flip($alphabet);
    $return_value = -1;
    $length = strlen($data);
    for ($i = 0; $i < $length; $i++) {
        $return_value +=
            ($alpha_flip[$data[$i]] + 1) * pow(26, ($length - $i - 1));
    }
    return $return_value;
}

Shortest way, in PHP >= 4.1.0

$alphabet = range('A', 'Z');

echo $alphabet[3]; // returns D

echo array_search('D', $alphabet); // returns 3