Is the word dentro the same in Brazilian and European Portuguese?

Yes

The word "dentro" is identical in meaning, spelling, and grammatical function in both Brazilian and Continental Portuguese. The only difference lies in pronunciation. In Brazilian Portuguese, vowels tend to be more open and clearly articulated. In Continental Portuguese, unstressed vowels are often more closed, neutralized, or even elided (reduced) depending on the regional accent.

Brazilian Portuguese Examples

1. O celular está dentro da bolsa. (The cell phone is inside the bag.)
2. Coloque o lixo dentro do saco. (Put the trash inside the bag.)
3. Ele está dentro da academia. (He is inside the gym.)
4. A comida está dentro da geladeira. (The food is inside the refrigerator.)
5. Não deixe o cachorro dentro de casa. (Don't leave the dog inside the house.)

Continental Portuguese Examples

1. O telemóvel está dentro da mala. (The mobile phone is inside the bag.)
2. Põe o lixo dentro do saco. (Put the trash inside the bag.)
3. Ele está dentro do ginásio. (He is inside the gym.)
4. A comida está dentro do frigorífico. (The food is inside the refrigerator.)
5. Não deixes o cão dentro de casa. (Don't leave the dog inside the house.)