Conexion OCI PHP

<?php
define ("USUARIO","own_tester");  //USUARIO DE LA BASE DE DATOS
define ("PASSWORD","123"); // CONTRASEÑA DE LA BASE DE DATOS
define ("HOST_INSTANCIA","localhost/orcl"); //HOST E INSTANCIA DEL SERVIDOR LOCALHOST/INSTANCIA
define ("CHARACTER_SET","AL32UTF8");

class Conexion{
private $conexion;

public function __construct(){ //Constructor
    $this->conexion = oci_connect(USUARIO, PASSWORD, HOST_INSTANCIA,CHARACTER_SET);
    if(!$this->conexion){
        echo "ERROR!!! AL CONECTAR AL SERVIDOR DE BASE DE DATOS";
    }
   $this->alter_sesion_date();

}
public function __destruct(){ //Destructor
oci_close($this->conexion);

}

public function query($sql){
 return oci_parse($this->conexion,$sql);

}
public function ejecutar($stm){
    return oci_execute($stm);


}
public function alter_sesion_date(){
$sesion = $this->query("alter session set nls_date_format='DD/MM/YYYY HH24:MI:SS'");
$this->ejecutar($sesion);

}



}
?>