Fixed Length Record I/O in PHP

Question

I have read a fixed length Record from the file Using php. I have given the code below.

$f = fopen('data.txt', 'rb');

while (!feof($f)) {
  // read one segment of 52 bytes
  if ($s = fread($f, 52)) {
    // unpack the binary structure into an associative array
    print_r(unpack('ieid/ieage/a20name/a20city', $s));
  }
}

fclose($f);

Now i need to know How to write the Fixed Length record to the file. I searched in many web sites but still i didn't get the correct answer . How can i write the fixed Length record to the binary / text file ?