Question

I’m a beginner at PHP, and I’m still trying to work out proper file handling techniques. I’m usually alright with trial and error, but when it comes to deleting and modifying data, I always like to be on the safe side.

I wrote the code below to delete a certain section of a file, but I’m not sure if it will work with larger files or under unforeseen conditions which require experience to code for.

I tested this just now and it did work, but I would like to run it by the more experienced programmers first:

function deletesection($start,$len){
    $pos=0;
    $tmpname=$this->name."tmp.tmp";
    $tmpf=fopen($tmpname,"wb+");
    rewind($tmpf);
    $h=fopen($this->name,"rb");
    rewind($h);
    while(!feof($h)){
        $this->xseek($h,$pos);
        $endpos = $pos+1000;
        if($endpos>$start && $pos<$start+$len){
            $readlen=$start-$pos;
            $nextpos=$start+$len;
        }
        else{
            $readlen=1000;
            $nextpos=$pos+1000;
        }
        fwrite($tmpf,fread($h,$readlen));
        $pos=$nextpos;
    }
    fclose($h);
    unlink($this->name);
    rename($tmpname,$this->name);
}

This is inside a class where the property “name” is the file path.

I’m writing the file 1000 bytes at a time because I was getting errors about the maximum amount of memory being exceeded when testing with files over 30mb.

Answer 1

I had a quick look at your code – seems a bit complicated, also copying the entire file will be less efficient if the section to delete is small in relation to the total filesize…

function deletesection($filename, $start, $len)
{
 $chunk=49128;
 if (!is_readable($filename) || !is_writeable($filename) || !is_file($filename)) {
   return false;
 }
 $tfile=tempnam(); // used to hold stuff after the section to delete
 $oh=fopen($tfile, 'wb');
 $ih=fopen($filename, 'rb');
 if (fseek($ih, $start+$len)) {
  while ($data=fgets($ih, $chunk) && !feof($ih) {
     fputs($oh,$data);
  }
  fclose($oh); $oh=fopen($tfile, 'rb'); 
       // or could just have opened it r+b to begin with
  fseek($ih, $start, SEEK_SET);
  while ($data=fgets($oh, $chunk) && !feof($oh) {
     fputs($ih, $data);
  }
 }
 fclose($oh);
 fclose($ih);
 unlink($tfile);
 return true;  
}

I believe it would also be possible to do this modifying the file in place (i.e. not using a second file) using a single file handle – but the code would get a bit messy and would require lots of seeks (then an ftruncate).

NB using files for managing data with PHP (and most other languages in a multi-user context) is not a good idea.