$fp_source = fopen('database.txt', 'r');
$fp_dest = fopen('database_temp.txt', 'w'); // better to generate a real temp filename
fwrite($fp_dest, 'new content');
while (!feof($fp_source)) {
    $contents .= fread($fp_source, 8192);
    fwrite($fp_dest, $contents);
}
fclose($fp_source);
fclose($fp_dest);
unlink('database.txt');
rename('database_temp.txt','database.txt');