What are some common challenges when updating from PHP 4.2 to PHP 5?

One common challenge when updating from PHP 4.2 to PHP 5 is the change in object-oriented programming syntax. In PHP 5, there are new features like visibility keywords (public, protected, private) and constructors (__construct instead of the class name). To solve this, you need to update your classes to adhere to the new syntax.

// PHP 4.2 syntax
class MyClass {
    var $myProperty;

    function MyClass() {
        $this->myProperty = 'value';
    }
}

// PHP 5 syntax
class MyClass {
    public $myProperty;

    public function __construct() {
        $this->myProperty = 'value';
    }
}