<?php
class User {
 
   protected $id;
   public $dados;
   public function __construct($id) {
      $this->id = $id; // aqui já tens o teu id
      // echo $this->id; // vai imprimir 4 e podes fazer o que quiseres com ele ao longo dos metodos/atributos desta instância
      // aceder à base de dados, SELECT * FROM users WHERE id = $id, 4 neste caso
 
      // depois já terás os dados que queres acerca do utilizador
      $this->dados = array('id' => 4, 'nome' => 'Miguel', 'email' => 'hdj@jdj.di');
   }
}
 
$u = new User(4);
echo $u->dados['email']; // hdj@jdj.di

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