PHP CookBook

Recipe 10.2 Using Text-File Databases

10.2.1 Problem

You want a lightweight way to store information between requests.

10.2.2 Solution

Use a text file with advisory locking to prevent conflicts. You can store data in the text file in any useful format (CSV, pipe-delimited, etc.) One convenient way is to put all the data you want to store in one variable (a big associative array) and then store the output of calling serialize( ) on the variable:

$data_file = '/tmp/data';

// open the file for reading and writing
$fh = fopen($data_file,'a+') or die($php_errormsg);
rewind($fh)                  or die($php_errormsg);

// get an exclusive lock on the file 
flock($fh,LOCK_EX)           or die($php_errormsg);

// read in and unserialize the data
$serialized_data = fread($fh,filesize($data_file)) or die($php_errormsg);
$data = unserialize($serialized_data);

/*
 * do whatever you need to with $data ...
 */

// reserialize the data 
$serialized_data = serialize($data);

// clear out the file
rewind($fh)                  or die($php_errormsg);
ftruncate($fp,0)             or die($php_errormsg);

// write the data back to the file and release the lock 
if (-1 == (fwrite($fh,$serialized_data))) { die($php_errormsg); }
fflush($fh)                  or die($php_errormsg);
flock($fh,LOCK_UN)           or die($php_errormsg);
fclose($fh)                  or die($php_errormsg);

10.2.3 Discussion

Storing your data in a text file doesn't require any additional database software to be installed, but that's pretty much its only advantage. Its main disadvantages are clumsiness and inefficiency. At the beginning of a request, you've got to lock your text file and haul out all your data from it, even if you're only using a little bit of the data. Until you unlock the file at the end of the request, all other processes have to wait around, doing nothing, which means all your users are waiting too. One of the great assets of databases is that they give you structured access to your data, so you only lock (and load into memory) the data you actually care about. The text file solution doesn't do that.

What's worse, the locking you can do with a text file isn't nearly as robust as what you can do with a database. Because flock( ) provides a kind of file locking called advisory locking, the only thing that prevents multiple processes from stepping on each other and trashing your data is politeness and diligent programming. There's no guarantee your data is safe from an innocently incompetent or intentionally malicious program.

10.2.4 See Also

Recipe 5.8 discusses serializing data; Recipe 18.25 goes into the details of file locking; documentation on flock( ) at http://www.php.net/flock, serialize( ) at http://www.php.net/serialize, and unserialize( ) at http://www.php.net/unserialize.