Question

a.php

<?php

echo "AAA\n";
if(class_exists('NotImplementedException')) {
    echo "BBB\n";
    return;
    echo "DDD\n";
}

echo "CCC\n";

/**
 * Thrown when a feature is not yet implemented.
 */
class NotImplementedException extends Exception {}

b.php

<?php
include 'a.php';
include 'a.php';

When I run b.php I get:

AAA
BBB
PHP Fatal error:  Cannot redeclare class NotImplementedException in /home/mark/Tests/IncludeTwice/a.php on line 15

What’s going on here? The output I would expect is

AAA
CCC
(class gets declared for first time)
AAA
BBB
(execution stops)

---

To those saying “the interpreter scans and parses the whole file first”: then why does this work?:

<?php

class A{}

if(!class_exists('A')) {
    class A{}
}

Answer 1

I think one part is a bug…

The PHP interpreter works differently if you are extending a class.

This works:

<?php

class a {}
exit();
class a extends Exception {}

?>

This does not:

<?php

class a extends Exception {}
exit();
class a {}

?>

From what I can see in the PHP 5.4.0 source, there are two different functions that handle the binding of classes: do_bind_class:4494 for standalone classes and do_bind_inherited_class:4533 for inherited classes (ones that use the extend keyword).

The stand-alone function has a conditional that ignores duplicate class definitions at compile time in case it is never hit at runtime. The inherited class version is missing this conditional (maybe on purpose, maybe not).

I would patch the inherited function to have the same conditional, test and submit to the developers.

And the other part is a feature…

As for your conditional class piece: I believe that PHP will load your class at compile-time if it is declared at global scope, but if it is contained inside a block, it will not be loaded until it is executed.

This works:

<?php

$a = new A();

//{
    class A {}
//}

?>

This doesn’t work, even though it is logically identical:

<?php

$a = new A();

{
    class A {}
}

?>

When you combine these two pieces of functionality, your problem arises since your class is both an inherited class and declared at global scope.