J. Mike Rollins: reference

J. Mike Rollins (Sparky) [rollins@wfu.edu]
CISSP, GIAC GPEN

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PHP/Perl Reference

I am proficient with Perl. More and more, PHP is used in many of the applications I use (Gallery2 and PHPBB2). My first real PHP program was an XML parser that used recursion and objects. I spent hours debugging the code. A better understanding of PHP references would have saved me much time.

The following code defines an object using Perl and PHP.

Perl

PHP

package MyTest;

use strict;

sub new
  {
    my $pkg = shift;

    my $obj = {
        'value' => 0,
        'is_null' => 1
      };
    bless $obj,$pkg;

    return $obj;
  }

sub setValue
  {
    my $obj = shift;
    my $value = shift;

    $obj->{'value'} = $value;
    $obj->{'is_null'} = 0;
  }

sub getValue
  {
    my $obj = shift;
    return $obj->{'value'};
  }

sub isNull
  {
    my $obj = shift;
    return $obj->{'is_null'};
  }

return 1;

<?php

class MyTest
  {
    var $value;
    var $is_null;

    function MyTest()
      {
        $this->value = 0;
        $this->is_null = true;
      }

    function setValue($value)
      {
        $this->value = $value;
        $this->is_null = false;
      }

    function getValue()
      {
        return $this->value;
      }

    function isNull()
      {
        return $this->is_null;
      }

    }
?>

Part 1

A Perl script an a PHP script will illustrate a subtle difference in the two languages.

Perl

PHP

#!/usr/bin/perl

use MyTest;

#
# Create new instance of MyTest()
#

my $a = new MyTest();

$a->setValue(10);

print "Create new object \$a\n";
print "A = " . $a->getValue() . "\n\n";

#
# Assign b to a
#

my $b = $a;

print "Assign \$b = \$a\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

#
# Setting b to 11
#

print "Setting \$b to 11\n";
$b->setValue(11);
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

exit(0);

<?php

require 'MyTest.php';

/*
 * Create new instance of MyTest()
 */

$a = new MyTest();

$a->setValue(10);

print "Create new object \$a\n";
print "A = " . $a->getValue() . "\n\n";

/*
 * Assign b to a
 */

$b = $a;

print "Assign \$b = \$a\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

/*
 * Setting b to 11
 */

$b->setValue(11);
print "Setting \$b to 11\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

?>

The output of these scripts show a major difference between Perl and PHP.

Perl	PHP
Create new object $a A = 10 Assign $b = $a A = 10 B = 10 Setting $b to 11 A = 11 B = 11	Create new object $a A = 10 Assign $b = $a A = 10 B = 10 Setting $b to 11 A = 10 B = 11

Part 2

PHP performs a "copy" of the object in response to the = operator. Perl creates a reference to the existing object. The =& operator will instruct PHP to create a reference to the existing object. But, the relationship with the reference is much stronger that in Perl.

The following modifies the PHP code from the previous example:

Perl

PHP

#!/usr/bin/perl

use MyTest;

#
# Create new instance of MyTest()
#

my $a = new MyTest();

$a->setValue(10);

print "Create new object \$a\n";
print "A = " . $a->getValue() . "\n\n";

#
# Assign b to a
#

my $b = $a;

print "Assign \$b = \$a\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

#
# Setting b to 11
#

print "Setting \$b to 11\n";
$b->setValue(11);
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

exit(0);

<?php

require 'MyTest.php';

/*
 * Create new instance of MyTest()
 */

$a = new MyTest();

$a->setValue(10);

print "Create new object \$a\n";
print "A = " . $a->getValue() . "\n\n";

/*
 * Assign b to a
 */

$b =& $a;

print "Assign \$b =& \$a\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

/*
 * Setting b to 11
 */

$b->setValue(11);
print "Setting \$b to 11\n";
print "A = " . $a->getValue() . "\n";
print "B = " . $b->getValue() . "\n\n";

?>

Now the output of these two scripts are the same.

Perl	PHP
Create new object $a A = 10 Assign $b = $a A = 10 B = 10 Setting $b to 11 A = 11 B = 11	Create new object $a A = 10 Assign $b =& $a A = 10 B = 10 Setting $b to 11 A = 11 B = 11

Part 3

Be careful, the =& operator creates a stronger association with the object than the = operator in Perl.

Perl

PHP

#
# Add the following to the Perl script
#

$a = 0;
print "Setting \$a=0\n";
print "A = " . $a . "\n";
print "B = " . $b->getValue() . "\n";

#
# This will display
#
# Setting $a=0
# A = 0
# B = 11
#

/*
 * Add the following code to the php script
 */

$a = 0;
print "Setting \$a=0\n";
print "A = " . $a . "\n";
print "B = " . $b->getValue() . "\n";

/*
 * This will produce the following
 *
 * Setting $a=0
 * A = 0
 * Fatal error:  Call to a member function on a non-object...
 *
 */

The assignment of $a=0 in the PHP code replaced the object that $a and $b referenced with the number 0. So, $b now references the number 0.