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Class and object changes

instanceof, is_a(), is_subclass_of() and catch

In PHP 5.0, is_a() was deprecated and replaced by the instanceof operator. There were some issues with the initial implementation of instanceof, which relied on __autoload() to search for missing classes. If the class was not present, instanceof would throw a fatal E_ERROR due to the failure of __autoload() to discover that class. The same behaviour occurred in the catch operator and the is_subclass_of() function, for the same reason.

None of these functions or operators call __autoload() in PHP 5.1.x, and the class_exists() workarounds used in code written for PHP 5.0.x, while not problematic in any way, are no longer necessary.

Abstract private methods

Abstract private methods were supported between PHP 5.0.0 and PHP 5.0.4, but were then disallowed on the grounds that the behaviours of private and abstract are mutually exclusive.

Access modifiers in interfaces

Under PHP 5.0, function declarations in interfaces were treated in exactly the same way as function declarations in classes. This has not been the case since October 2004, at which point only the public access modifier was allowed in interface function declarations. Since April 2005 - which pre-dates the PHP 5.0b1 release - the static modifier has also been allowed. However, the protected and private modifiers will now throw an E_ERROR, as will abstract. Note that this change should not affect your existing code, as none of these modifiers makes sense in the context of interfaces anyway.

Changes in inheritance rules

Under PHP 5.0, it was possible to have a function declaration in a derived class that did not match the declaration of the same function in the base class, e.g.

This code will cause an E_STRICT error to be emitted under PHP 5.1.x.

class Base {
    function &return_by_ref() {
        $r = 1;
        return $r;
class Derived extends Base {
    function return_by_ref() {
        return 1;

Class constants

Under PHP 5.0.x, the following code was valid:

Under PHP 5.1.x, redefinition of a class constant will throw a fatal E_ERROR.

class test {
    const foobar = 'foo';
    const foobar = 'bar';

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