read single byte from PHP uploaded file

I am very new to PHP, so forgive me my ignorance.

I have HTML for for upload:

<form action="upload.php" method="post" enctype="multipart/form-data">
<label for="file">Filename:</label>
<input type="file" name="file" id="file" /><br />
<input type="submit" name="submit" value="Submit" />
</form>

My PHP is (It doens't know, but hopefully it shows what I want):

<?php
if ($_FILES["file"]["error"] > 0)
  {
  echo "Error: " . $_FILES["file"]["error"] . "<br />";
  }
else
  {
  $content = fread($_FILES["file"],10);
  echo $content;
  echo "byte10 is: " ???
  }
?>

I want to: From this uploaded file, I want to read a single byte (say byte 10). I want to print out the acscii (HEX) code for this byte. How do I do that? Do I need to save the file to the server?

开发者_如何学Python(The untimate goal is to encrypt the file and send th eencrypted file back to the user. So I want (1) upload file (2) read each individual byte (2) perform encryption on byte level (3) save file and send it back to the user)

---

Multiple implementations, I'm gonna give you a reusable one, using fseek.

<?php
function getByte($f,$n){
    fseek($f,$n);
    return fread($f,1);
}

$f=fopen('your_file_path');
echo getByte($f,10);
?>

I strongly suggest using move_uploaded_file before using anything $_FILES.

P.S. This can get very very expensive if you're not careful, my suggestion to you, would be to read bigger chunks and then perform operations on them.

---

Thank you very much for the "ultimate goal" -- it will help us make sure that you don't do something silly by accident.

Encryption is a very complex topic. You should not write your own encryption routine. Use a complete, pre-built solution.

Mcrypt is PHP's best encryption extension. Mcrypt supports multiple common ciphers including AES ("Rijndael"). Encryption is pretty darn easy -- use "CBC" mode for your file.

---

The Vigenère cipher is a 460-ish year old substitution cipher based on a simple lookup table.

This explains the whole one-character-at-a-time thing.

While you can read the whole file one byte at a time, you might find it more convenient to work with a string. You can read the entire file into a variable (through file_get_contents or whatever), and then use substr or the square bracket substring syntax to read the string one byte at a time. Remember, PHP strings are just simple lists of bytes.

Given $data is the complete data in the file, you can process every byte thusly:

$length = strlen($data);
for($i = 0; $i < $length; $i++) {
    $byte = $data[$i];
// Or
    $byte = substr($data, $i, 1);
// Process the byte here.
}

This is only practical for small files (smaller than a few megabytes), as it requires loading the whole file into memory.

I want to print out the acscii (HEX) code for this byte

You can get the decimal value using ord, then convert that to a hexadecimal string using dechex:

$hex_value = dechex(ord($byte));

Post-substitution, you can reverse it -- use hexdec to go from hex to decimal, then chr to go from decimal to byte:

$converted_byte = chr(hexdec($ciphered_hex));

---

In PHP, a string is basically an array of bytes, so you can use the fread() function to read some characters in to a string, and then access individual bytes using array notation. Eg.

$content = fread($_FILES["file"],10);
echo $content;
echo "byte10 is: ".$content[9]; // 9 because offset starts from 0