PHP Manual: Examples

Examples

We define 3 classes, each implementing a different storage method:

Example #1 storage_classes.inc


<?php
class FileStorage {
   var $data;

   function FileStorage($data) 
   {
       $this->data = $data;
   }
   
   function write($name) 
   {
       $fp = fopen(name, "w");
       fwrite($fp, $this->data);
       fclose($data);
   }
}

class WDDXStorage {
   var $data;
   var $version = "1.0";
   var $_id; // "private" variable

   function WDDXStorage($data) 
   {
       $this->data = $data;
       $this->_id = $this->_genID();
   }

   function store() 
   {
       if ($this->_id) {
           $pid = wddx_packet_start($this->_id);
           wddx_add_vars($pid, "this->data");
           $packet = wddx_packet_end($pid);
       } else {
           $packet = wddx_serialize_value($this->data);
       }
       $dbh = dba_open("varstore", "w", "gdbm");
       dba_insert(md5(uniqid("", true)), $packet, $dbh);
       dba_close($dbh);
   }

   // a private method
   function _genID() 
   {
       return md5(uniqid(rand(), true));
   }
}

class DBStorage {
   var $data;
   var $dbtype = "mysql";

   function DBStorage($data) 
   {
       $this->data = $data;
   }

   function save() 
   {
       $dbh = mysql_connect();
       mysql_select_db("storage", $dbh);
       $serdata = serialize($this->data);
       mysql_query("insert into vars ('$serdata',now())", $dbh);
       mysql_close($dbh);
   }
}

?>

We then instantiate a couple of objects from the defined classes, and perform some aggregations and deaggregations, printing some object information along the way:

Example #2 test_aggregation.php


<?php
include "storageclasses.inc";

// some utilty functions

function p_arr($arr) 
{
   foreach ($arr as $k => $v)
       $out[] = "\t$k => $v";
   return implode("\n", $out);
}

function object_info($obj) 
{
   $out[] = "Class: " . get_class($obj);
   foreach (get_object_vars($obj) as $var=>$val) {
       if (is_array($val)) {
           $out[] = "property: $var (array)\n" . p_arr($val);
       } else {
           $out[] = "property: $var = $val";
       }
   }
   foreach (get_class_methods($obj) as $method) {
       $out[] = "method: $method";
   }
   return implode("\n", $out);
}


$data = array(M_PI, "kludge != cruft");

// we create some basic objects
$fs = new FileStorage($data);
$ws = new WDDXStorage($data);

// print information on the objects
echo "\$fs object\n";
echo object_info($fs) . "\n";
echo "\n\$ws object\n";
echo object_info($ws) . "\n";

// do some aggregation

echo "\nLet's aggregate \$fs to the WDDXStorage class\n";
aggregate($fs, "WDDXStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";

echo "\nNow let us aggregate it to the DBStorage class\n";
aggregate($fs, "DBStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";

echo "\nAnd finally deaggregate WDDXStorage\n";
deaggregate($fs, "WDDXStorage");
echo "\$fs object\n";
echo object_info($fs) . "\n";

?>

We will now consider the output to understand some of the side-effects and limitation of object aggregation in PHP. First, the newly created $fs and $ws objects give the expected output (according to their respective class declaration). Note that for the purposes of object aggregation, private elements of a class/object begin with an underscore character ("_"), even though there is not real distinction between public and private class/object elements in PHP.

$fs object
Class: filestorage
property: data (array)
   0 => 3.1415926535898
   1 => kludge != cruft
method: filestorage
method: write

$ws object
Class: wddxstorage
property: data (array)
   0 => 3.1415926535898
   1 => kludge != cruft
property: version = 1.0
property: _id = ID::9bb2b640764d4370eb04808af8b076a5
method: wddxstorage
method: store
method: _genid

We then aggregate $fs with the WDDXStorage class, and print out the object information. We can see now that even though nominally the $fs object is still of FileStorage, it now has the property $version, and the method store, both defined in WDDXStorage. One important thing to note is that it has not aggregated the private elements defined in the class, which are present in the $ws object. Also absent is the constructor from WDDXStorage, which will not be logical to aggegate.

Let's aggregate $fs to the WDDXStorage class
$fs object
Class: filestorage
property: data (array)
   0 => 3.1415926535898
   1 => kludge != cruft
property: version = 1.0
method: filestorage
method: write
method: store

The process of aggregation is cumulative, so when we aggregate $fs with the class DBStorage, generating an object that can use the storage methods of all the defined classes.

Now let us aggregate it to the DBStorage class
$fs object
Class: filestorage
property: data (array)
   0 => 3.1415926535898
   1 => kludge != cruft
property: version = 1.0
property: dbtype = mysql
method: filestorage
method: write
method: store
method: save

Finally, the same way we aggregated properties and methods dynamically, we can also deaggregate them from the object. So, if we deaggregate the class WDDXStorage from $fs, we will obtain:

And deaggregate the WDDXStorage methods and properties
$fs object
Class: filestorage
property: data (array)
   0 => 3.1415926535898
   1 => kludge != cruft
property: dbtype = mysql
method: filestorage
method: write
method: save

One point that we have not mentioned above, is that the process of aggregation will not override existing properties or methods in the objects. For example, the class FileStorage defines a $data property, and the class WDDXStorage also defines a similar property which will not override the one in the object acquired during instantiation from the class FileStorage.