PHP Files Modifying a File in Place Without a Temporary File

PHP Files 

Modifying a File in Place Without a Temporary File

Problem

You want to change a file without using a temporary file to hold the changes.

Solution

Read the file with file_get_contents(), make the changes, and rewrite the file with file_put_contents():

       $contents = file_get_contents('pickles.txt');

       $contents = strtoupper($contents);

       file_put_contents('pickles.txt', $contents);

Discussion

This example turns text emphasized with asterisks or slashes into text with HTML <b> or <i> tags:

       $contents = file_get_contents('message.txt');

       // convert *word* to <b>word</b>

       $contents = preg_replace('@\*(.*?)\*@i','<b>$1</b>',$contents);

       // convert /word/ to <i>word</i>

       $contents = preg_replace('@/(.*?)/@i','<i>$1</i>',$contents);

       file_put_contents('message.txt', $contents);

Because adding HTML tags makes the file grow, the entire file has to be read into memory and then processed. If the changes to a file make each line shrink (or stay the same size), the file can be processed line by line, saving memory.

For example, this converts text marked with <b> and <i> to text marked with asterisks and slashes:

       $fh = fopen('message.txt','r+')                 or die($php_errormsg);

       // figure out how many bytes to read

       $bytes_to_read = filesize('message.txt');

       // initialize variables that hold file positions

       $next_read = $last_write = 0;

       // keep going while there are still bytes to read

       while ($next_read < $bytes_to_read) {

              /* move to the position of the next read, read a line, and save

              * the position of the next read */

              fseek($fh,$next_read);

              $s = fgets($fh)                                            or die($php_errormsg);

              $next_read = ftell($fh);

              // convert <b>word</b> to *word*

              $s = preg_replace('@<b[^>]*>(.*?)</b>@i','*$1*',$s);

              // convert <i>word</i> to /word/

              $s = preg_replace('@<i[^>]*>(.*?)</i>@i','/$1/',$s);

              /* move to the position where the last write ended, write the

              * converted line, and save the position for the next write */

              fseek($fh,$last_write);

              if (-1 == fwrite($fh,$s))                            { die($php_errormsg); }

              $last_write = ftell($fh);

       }

       

       // truncate the file length to what we've already written

       ftruncate($fh,$last_write)                              or die($php_errormsg);

       // close the file

       fclose($fh)                                                          or die($php_errormsg);