What are the differences between PHP4 and PHP5 that can cause issues with scripts?

One major difference between PHP4 and PHP5 is the introduction of new object-oriented features in PHP5, such as visibility keywords (public, private, protected) and magic methods (__construct, __destruct). If your PHP4 script relies on object-oriented programming without adhering to these new features, it may cause compatibility issues when running on PHP5. To solve this, update your code to use the proper visibility keywords and magic methods.

class MyClass {
    private $myProperty;

    public function __construct() {
        // constructor code
    }

    public function myMethod() {
        // method code
    }
}