What are the advantages of using getters and setters instead of functions or simply public fields in PHP?

I'm not a PHP developer, so I'm wondering what the advantages and disadvantages are in PHP to using explicit getter/setters, in a pure OOP style, with private fields (the way I like): class MyClass { private $firstField; private $secondField; public function getFirstField() { return $this->firstField; } public function setFirstField($x) { $this->firstField = $x; } public function getSecondField() { return $this->secondField; } public function setSecondField($x) { $this->secondField = $x; } } or just public fields: class MyClass { public $firstField; public $secondField; }