PHP Cookbook

18.16. 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 into memory, make the changes, and rewrite the file. Open the file with mode r+ (rb+, if necessary, on Windows) and adjust its length with ftruncate( ) after writing out changes:

// open the file for reading and writing 
$fh = fopen('pickles.txt','r+')         or die($php_errormsg);

// read the entire file into $s
$s = fread($fh,filesize('pickles.txt')) or die($php_errormsg);

// ... modify $s ...

// seek back to the beginning of the file and write the new $s
rewind($fh);
if (-1 == fwrite($fh,$s))                { die($php_errormsg); }

// adjust the file's length to just what's been written
ftruncate($fh,ftell($fh))               or die($php_errormsg);

// close the file
fclose($fh)                             or die($php_errormsg);

Discussion

The following code turns text emphasized with asterisks or slashes into text with HTML <b> or <i> tags:

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

// read the entire file into $s
$s = fread($fh,filesize('message.txt')) or die($php_errormsg);

// convert *word* to <b>word</b>
$s = preg_replace('@\*(.*?)\*@i','<b>$1</b>',$s);
// convert /word/ to <i>word</i>
$s = preg_replace('@/(.*?)/@i','<i>$1</i>',$s);

rewind($fh);
if (-1 == fwrite($fh,$s))                { die($php_errormsg); }
ftruncate($fh,ftell($fh))               or die($php_errormsg);
fclose($fh)                             or die($php_errormsg);

Because adding HTML tags makes the file grow, the entire file has to be read into memory ...