I just got through tracking down a very pernicious bug.

The problem was that an array assigned to an object was retaining its original link after cloning.  The original array was created with its values being assigned by reference.  This wouldn’t have been a problem in and of itself, but additionally there was a second array being created with a reference to the same variable.  This was both unnecessary and certain death to my script.

The takeaway from this is to avoid unnecessary variable assignment by reference. The original code was written over two years ago.

If I recall correctly my justification for using references was to save memory.

This is wrong.

Don’t think about references in a context of memory consumption (especially since PHP allocates on write), think about references in a context of “How do I need to treat this data?”

Here is the full script that is broken. Notice in the foreach two referential assignments.

$in = array( 'a' => '123', 'b' => '456' );
$vars = array();

foreach( $in as $key => $val )
{
	$_GET[$key] =& $val;
	$vars[] =& $val;
	unset( $val );
}

class test
{
	protected $_data;

	public function set( $data )
	{
		$this->_data = $data;
	}

	public function setVar( $key, $val )
	{
		$this->_data[$key] = $val;
	}
}

header( 'content-type: text/plain' );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

$test = new test();
$test->set( $_GET );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

$second = clone $test;
$second->setVar( 'a', 'abc' );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

Output

Should be 123 and 456
array(2) {
  ["a"]=>
  &string(3) "123"
  ["b"]=>
  &string(3) "456"
}
Should be 123 and 456
array(2) {
  ["a"]=>
  &string(3) "123"
  ["b"]=>
  &string(3) "456"
}
Should be 123 and 456
array(2) {
  ["a"]=>
  &string(3) "abc"
  ["b"]=>
  &string(3) "456"
}

Corrected Script

$in = array( 'a' => '123', 'b' => '456' );

foreach( $in as $key => $val )
{
	$_GET[$key] = $val;
}

class test
{
	protected $_data;

	public function set( $data )
	{
		$this->_data = $data;
	}

	public function setVar( $key, $val )
	{
		$this->_data[$key] = $val;
	}
}

header( 'content-type: text/plain' );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

$test = new test();
$test->set( $_GET );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

$second = clone $test;
$second->setVar( 'a', 'abc' );

echo 'Should be 123 and 456' . PHP_EOL;
var_dump( $_GET );

Correct Output

Should be 123 and 456
array(2) {
  ["a"]=>
  string(3) "123"
  ["b"]=>
  string(3) "456"
}
Should be 123 and 456
array(2) {
  ["a"]=>
  string(3) "123"
  ["b"]=>
  string(3) "456"
}
Should be 123 and 456
array(2) {
  ["a"]=>
  string(3) "123"
  ["b"]=>
  string(3) "456"
}

I’ve learned a lot about PHP5 OOP visibility recently.  I’m posting a test script that will share some of my discoveries.

Take away:

1. Private really does mean private.
2. Visibility keywords are honored while cloning
3. PHP does not enforce the visibility keywords on variables using a fatal error.
   It uses an E_NOTICE : Notice: Undefined property

define( 'SCRIPT_EOL', '<br>' );

abstract class a_base_class
{
    private $name = 'Johannes Kepler';
    protected $color = 'Green';

    public function __set($name, $value)
    {
        throw new Exception( 'a_base_class - Undefined or private property: ' . $name );
    }

    public function __clone()
    {
        // this does work - $this->name is private but is accessible in this class
        $this->name = 'Isaac Newton';
    }

    public function getName()
    {
        return $this->name;
    }

    public function echoName()
    {
        echo $this->name;
    }
}

class exampleClass1 extends a_base_class
{
    function __clone()
    {
        // this does not work - $this->name is private to ac and can't be modified here
        $this->name = 'Isaac Newton';
    }

    function echoName()
    {
        // this will issue an E_NOTICE because the variable is undefined in this class (private variables do not carry through to descendants)
        echo $this->name;
    }

    function echoColor()
    {
        echo $this->color;
    }
}

class exampleClass2 extends a_base_class
{
    public function __clone()
    {
        $this->color = 'Red';
        // without this, $this->name will not get changed to Isaac Newton
        parent::__clone();
    }

    public function echoColor()
    {
        echo $this->color;
    }
}

echo '</pre>

<hr />

<pre>
';
echo '</pre>
<h2>exampleClass1</h2>
<pre>
';
$instance = new exampleClass1();
echo 'calling echoName()' . SCRIPT_EOL;
$instance->echoName();
echo 'echo $instance->name' . SCRIPT_EOL;
echo $instance->name . SCRIPT_EOL;
echo SCRIPT_EOL;
echo 'echoing getName()' . SCRIPT_EOL;
echo $instance->getName(); // calling public method on base class
echo SCRIPT_EOL;
echo 'calling echoColor()' . SCRIPT_EOL;
$instance->echoColor();
echo SCRIPT_EOL;

try
{
    $newInstance = clone $instance;
}
catch( Exception $e )
{
    echo SCRIPT_EOL . '------------------------------------------------' . SCRIPT_EOL;
    echo "Exception Thrown: " . $e->getMessage();
    echo SCRIPT_EOL;
echo '
';
    echo $e->getTraceAsString();
    echo '

';
    echo SCRIPT_EOL . '------------------------------------------------' . SCRIPT_EOL;
}
echo SCRIPT_EOL;
echo 'var_dump' . SCRIPT_EOL;
var_dump( $newInstance );

echo '</pre>

<hr />

<pre>
';
echo '</pre>
<h2>exampleClass2</h2>
<pre>
';

$instance = new exampleClass2();
echo 'calling echoName()' . SCRIPT_EOL;
$instance->echoName();
echo SCRIPT_EOL;
echo 'echo $instance->name' . SCRIPT_EOL;
echo $instance->name . SCRIPT_EOL;
echo 'echoing getName()' . SCRIPT_EOL;
echo $instance->getName(); // calling public method on base class
echo SCRIPT_EOL;
echo 'calling echoColor()' . SCRIPT_EOL;
$instance->echoColor();
echo SCRIPT_EOL;

try
{
    $newInstance = clone $instance;
}
catch( Exception $e )
{
    echo SCRIPT_EOL . '------------------------------------------------' . SCRIPT_EOL;
    echo "Exception Thrown: " . $e->getMessage();
    echo SCRIPT_EOL;
    echo $e->getTraceAsString();
    echo SCRIPT_EOL . '------------------------------------------------' . SCRIPT_EOL;
}

echo '
';
var_dump( $newInstance );
echo '

';

Output

---

exampleClass1

calling echoName()

Notice: Undefined property: exampleClass1::$name in /Users/timgallagher/Desktop/xampp/se/php5oop.php on line 43
echo $instance->name

Notice: Undefined property: exampleClass1::$name in /Users/timgallagher/Desktop/xampp/se/php5oop.php on line 73

echoing getName()
Johannes Kepler
calling echoColor()
Green

————————————————
Exception Thrown: a_base_class – Undefined or private property: name

#0 /Users/timgallagher/Desktop/xampp/se/php5oop.php(37): a_base_class->__set('name', 'Isaac Newton')
#1 /Users/timgallagher/Desktop/xampp/se/php5oop.php(84): exampleClass1->__clone()
#2 {main}

————————————————

var_dump

Notice: Undefined variable: newInstance in /Users/timgallagher/Desktop/xampp/se/php5oop.php on line 98
NULL

---

exampleClass2

calling echoName()
Johannes Kepler
echo $instance->name

Notice: Undefined property: exampleClass2::$name in /Users/timgallagher/Desktop/xampp/se/php5oop.php on line 108

echoing getName()
Johannes Kepler
calling echoColor()
Green

object(exampleClass2)#1 (2) {
  ["name:private"]=>
  string(12) "Isaac Newton"
  ["color:protected"]=>
  string(3) "Red"
}