What are the differences in object-oriented programming between PHP4 and PHP5 that could affect script compatibility?

In PHP5, one major difference in object-oriented programming is the introduction of visibility keywords (public, private, protected) to control access to class properties and methods. This means that code written in PHP4 that relies on direct access to class properties or methods without regard to visibility may not work in PHP5. To fix this issue, update the code to adhere to proper visibility rules by using the appropriate keywords for class properties and methods. 

class MyClass {
    private $property;

    public function getProperty() {
        return $this->property;
    }

    public function setProperty($value) {
        $this->property = $value;
    }
}

Keywords

PHP4 PHP5 object-oriented programming script compatibility differences

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