PHP Files Modifying a File in Place Without a Temporary File - Web Development and Design | Tutorial for Java, PHP, HTML, Javascript PHP Files Modifying a File in Place Without a Temporary File - Web Development and Design | Tutorial for Java, PHP, HTML, Javascript

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Thursday, July 11, 2019

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);

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