foo.php

<?php
class Persona
{
    public $nombres = '';
    public $ape_paterno = '';
    public $ape_materno = '';
    public $edad = null;

    public function ponerNombresApellidos($nombres, $ape_pat, $ape_mat)
    {
        $this->nombres = $nombres;
        $this->ape_paterno = $ape_pat;
        $this->ape_materno = $ape_mat;
    }

}

class Paciente extends Persona
{
    public $enfermedades = array();

    public function agregarEnfermedad($enfermedad)
    {
        $this->enfermedades[] = $enfermedad;
    }
}