<?php
$reading = fopen('file.txt', 'r');
$writing = fopen('file.tmp', 'w');

$replaced = false;

while (!feof($reading)) {
  $line = fgets($reading);
  if (stristr($line,'#id:4 ')) {
    $line = "replacement line!\n";
    $replaced = true;
  }
  fputs($writing, $line);
}
fclose($reading); fclose($writing);
if ($replaced)
{
  rename('file.tmp', 'file.txt');
} else {
  unlink('file.tmp');
}
?>