How to display errors in the form

Question

I’m working with the design standard (PRG) Post Redirect Get).

I have 3 files

1 - Class Pessoa.PHP

2 - formulario.PHP

3 - processa_form.PHP

What is the best way to handle and display errors in a web application?

Follow an example

class Pessoa {
   private nome;
   private ano_nasc;
   private cpf;

   public function getAno_nasc(){
       $this->ano_nasc;
   }
   public function getNome()....

the problem begins with the methods set

What is the best way to return errors from set and insert to the page containing the form?

I thought of the following possibilities

1 - I found it ineffective because I won’t know for sure the reason for the error

public function setAno_nasc($ano){
        if ($ano > 3000 || $ano < 1900 )
            return false;
        else
            return true;

2 - I thought it better but I still think that in the future it can be a problem because I’ll have to unite all the mistakes of set and then display on the form

public function setAno_nasc($ano){
        if ($ano > 3000 || $ano < 1900 ){
       return "Ano inválido";
   }
public function setCpf($cpf){
        if (strlen($cpf) > 11)
           return "CPF inválido";    
}    

3 - This way I thought better but I do not know if it is correct

public function setAno_nasc($ano){
        if ($ano > 3000 || $ano < 1900 ){
             $_SESSION['erro'] .= "Ano inválido#";
        }
}

public function setCpf($cpf){
        if (strlen($cpf) > 11)
           $_SESSION['erro'] .= "CPF inválido#";    
}

The operation is as follows:

1- The person accesses the processa_formulario.php and send the data to processa_form.php

2- The processa_form.php instance a person and arrow the data coming from the formulario.php in attributes, check that everything is all right and saved in the bank after redirects to the formulario.php with errors or with the message from success.

3- The formulario.php displays whether you were able to register or not, if no, it informs the problems so that they can be corrected

I have gone a little long so that there is no doubt in the understanding of the problem. This is a summary model of my problem, my person class has more than 50 attributes. (Use a dynamic function to set all class attributes)