Problem with PDO and query with variables

I have a query that does not run correctly when I enter it in PHP:

SELECT id_continente,
       id_pais,
       id_localidad,
       visitas,
       posicion_pais,
       posicion_continente
FROM  (SELECT *,
              @posicion := IF(@continente_actual = id_continente, @posicion + 1,
                           1) AS
              posicion_continente,
              @continente_actual := id_continente
       FROM   (SELECT *,
                      @puesto := IF(@pais_actual = id_pais, @puesto + 1, 1) AS
                      posicion_pais,
                      @pais_actual := id_pais
               FROM   lugar_localidades_top_site
               WHERE  id_site = :site
                      AND id_pais <> :pais
                      AND categoria >= :categoria
               ORDER  BY id_pais,
                         visitas DESC) rank_limitado_pais
       WHERE  posicion_pais <= :loc_por_pais
       ORDER  BY id_continente,
                 visitas DESC) rank_limitado_continente
WHERE  posicion_continente <= :loc_por_continente
ORDER  BY visitas DESC
LIMIT  0, 10  

It turns out that PDO interprets the sign: = as if it were a variable to declare and by not assigning it a value simply the obvious one and the query is executed as if it were the following:

SELECT id_continente,
       id_pais,
       id_localidad,
       visitas,
       posicion_pais,
       posicion_continente
FROM  (SELECT *,
              @posicion = IF(@continente_actual = id_continente, @posicion + 1,
                          1) AS
              posicion_continente,
              @continente_actual = id_continente
       FROM   (SELECT *,
                      @puesto = IF(@pais_actual = id_pais, @puesto + 1, 1) AS
                      posicion_pais,
                      @pais_actual = id_pais
               FROM   lugar_localidades_top_site
               WHERE  id_site = :site
                      AND id_pais <> :pais
                      AND categoria >= :categoria
               ORDER  BY id_pais,
                         visitas DESC) rank_limitado_pais
       WHERE  posicion_pais <= :loc_por_pais
       ORDER  BY id_continente,
                 visitas DESC) rank_limitado_continente
WHERE  posicion_continente <= :loc_por_continente
ORDER  BY visitas DESC
LIMIT  0, 10  

So it does not cause an error but it does not generate the indexes I need for the query to work properly, any ideas to solve it?