Efficient way for manipulating binary data

Question

I have a string containing bitmap data. Basically, it holds cycles of 1 byte of red, green, blue for each pixel in an image.

I want to manipulate the 8-bit integer value of each color channel in the bitmap. Currently I do this using unpack('C'.strlen($string)) which gives me an array of integers. This is very slow as it converts a lot of data.

Is there a more efficient way to access and modify data from a string as integers?

By more efficient, I mean more efficient than this:

for($i = 0, $l = strlen($string); $i < $l; $i++)
    $string[$i] = chr(floor(ord($string[$i]) / 2));

The manipulation where I divide by 2 is simply an example. The manipulations would be more advanced in a real example.

The problem with the above example is that for say 1 million pixels, you get 3 million function calls initialized from PHP (chr, floor, ord). On my server, this amounts to about 1.4 seconds for an example picture.

Answer 1

PHP strings essentially are byte arrays. The fastest way would simply be:

for ($i = 0, $length = strlen($data); $i < $length; $i++) {
    $byte = $data[$i];

    ...
}

If you manipulate the byte with bitwise operators, you'll hardly get more efficient than that. You can write the byte back into the string with $data[$i] = $newValue. If you need to turn a single byte into an int, use ord(), the other way around use chr().