如何将部分块写入现有文件？

I need to upload data from multiple sources into a single file, the chunks come in whatever order and I'm having trouble actually writing them into the file.

Here's an example:

function create_file($path, $size_b) {
    $f = fopen($path, "w");
    fseek($f, $size_b - 1, SEEK_CUR);
    fwrite($f, "0");
    fclose($f);
}

function write_file_chunk($path, $offset, $chunk) {
    $f = fopen($path, "w");
    fseek($f, $offset, SEEK_CUR);
    fwrite($f, $chunk);
    fclose($f);
}

$name = "boo.txt";
create_file($name, 128);
write_file_chunk($name, 10, "hello");
write_file_chunk($name, 0, "hello");

The problem is that fwrite just writes the string to where instructed and then just ends the file there regardless of whether there was data after it or not.

Is there a way to make fwrite not touch the rest of the file so I can fill out the file in a random order?

fopen mode w truncates the file to zero length.

Use r+, as this user comment on fseek suggests, http://php.net/manual/en/function.fseek.php#81450:

if you're using fseek() to write data to a file, remember to open the file in "r+" mode, example:

$fp=fopen($filename,"r+");

DON'T open the file in mode "a" (for append), because it puts the file pointer at the end of the file and doesn't let you fseek earlier positions in the file (it didn't for me!). Also, don't open the file in mode "w" -- although this puts you at the beginning of the file -- because it wipes out all data in the file.