Help! Replacing parts of PHP files

Question

My apologies if this is a dumb question.
I'm trying to copy a file and change a a small section of that file. How can I change that small section?
I tried doing so with by converting the byte array into a string, but how do you convert a string back into a byte array? Is there a better way of doing this?
Thanks for your help (I've been working on this problem all day :-( )
  public static void copyFile(File in, File out, int q) throws Exception {
    FileInputStream fis  = new FileInputStream(in);
    FileOutputStream fos = new FileOutputStream(out);
    byte[] buf = new byte[1024];
    int i = 0;
    String temp = "";
    while((i=fis.read(buf))!=-1) {
          temp = buf.toString();
          String explicitid = "explicitid=" + name[q];
          temp.replaceAll("explicitid=8", explicitid);
      fos.write(buf, 0, i);
    fis.close();
    fos.close();
  }

Answer 1

Let the I/O classes do the byte[]/String conversions for you; that's what they're there for. Another problem with your code is that you aren't doing anything with the result of the replaceAll() - it doesn't change the original string, it creates a new one. Try this (adapted from code on this page):
    public static void copyFile(File in, File out, int q) throws IOException {
        BufferedReader reader = new BufferedReader(new FileReader(in));
        BufferedWriter writer = new BufferedWriter(new FileWriter(out));
        //... Loop as long as there are input lines.
        String line = null;
        while ((line=reader.readLine()) != null) {
            writer.write(line.replaceAll("explicitid=8", "explicitid=" + name[q]));
            writer.newLine();   // Write system dependent end of line.
        //... Close reader and writer.
        reader.close();  // Close to unlock.
        writer.close();  // Close to unlock and flush to disk.
    }