10.3.1. Problem

10.3.2. Solution

$dbh = dba_open('fish.db','c','gdbm') or die($php_errormsg);

// retrieve and change values
if (dba_exists('flounder',$dbh)) {
  $flounder_count = dba_fetch('flounder',$dbh);
  $flounder_count++;
  dba_replace('flounder',$flounder_count);
  print "Updated the flounder count.";
} else {
  dba_insert('flounder',1);
  print "Started the flounder count.";
}

// no more tilapia
dba_delete('tilapia',$dbh);

// what fish do we have?
for ($key = dba_firstkey($dbh);  $key !== false; $key = dba_nextkey($dbh)) {
   $value = dba_fetch($key);
   print "$key: $value\n";
}

dba_close($dbh);

10.3.3. Discussion

PHP can support a few different kinds of DBM backends: GDBM, NDBM, DB2, DB3, DBM, and CDB. The DBA abstraction layer lets you use the same functions on any DBM backend. All these backends store key/value pairs. You can iterate through all the keys in a database, retrieve the value associated with a particular key, and find if a particular key exists. Both the keys and the values are strings.

$user = $_SERVER['argv'][1];
$password = $_SERVER['argv'][2];

$data_file = '/tmp/users.db';

$dbh = dba_open($data_file,'c','gdbm') or die("Can't open db $data_file");

if (dba_exists($user,$dbh)) {
    print "User $user exists. Changing password.";
} else {
    print "Adding user $user.";
}

dba_replace($user,$password,$dbh) or die("Can't write to database $data_file");

dba_close($dbh);

$data_file = '/tmp/users.db';
$total_length = 0;
if (! ($dbh = dba_open($data_file,'r','gdbm'))) {
    die("Can't open database $data_file");
}

$k = dba_firstkey($dbh);
while ($k) {
    $total_length += strlen(dba_fetch($k,$dbh));
    $k = dba_nextkey($dbh);
}

print "Total length of all passwords is $total_length characters.";

dba_close($dbh);

The dba_firstkey( ) function initializes $k to the first key in the DBM file. Each time through the while loop, dba_fetch( ) retrieves the value associated with key $k and $total_length is incremented by the length of the value (calculated with strlen( )). With dba_nextkey( ), $k is set to the next key in the file.

$dbh = dba_open('users.db','c','gdbm') or die($php_errormsg);

// read in and unserialize the data
if ($exists = dba_exists($_REQUEST['username'])) {
    $serialized_data = dba_fetch($_REQUEST['username']) or die($php_errormsg);
    $data = unserialize($serialized_data);
} else {
    $data = array();
}

// update values 
if ($_REQUEST['new_password']) {
    $data['password'] = $_REQUEST['new_password'];
}
$data['last_access'] = time();

// write data back to file
if ($exists) {
    dba_replace($_REQUEST['username'],serialize($data));
} else {
    dba_insert($_REQUEST['username'],serialize($data));
}

dba_close($dbh);

While this example can store multiple users' data in the same file, you can't search, for example, a user's last access time, without looping through each key in the file. Structured data like this belongs in a SQL database.

10.3.4. See Also