Hi,

First query the API configuration details : see here https://developer.themoviedb.org/reference/configuration-details

"images": {
    "base_url": "http://image.tmdb.org/t/p/",
    "secure_base_url": "https://image.tmdb.org/t/p/",
    "backdrop_sizes": [
      "w300",
      "w780",
      "w1280",
      "original"
    ],
    "logo_sizes": [
      "w45",
      "w92",
      "w154",
      "w185",
      "w300",
      "w500",
      "original"
    ],
    "poster_sizes": [
      "w92",
      "w154",
      "w185",
      "w342",
      "w500",
      "w780",
      "original"
    ],
    "profile_sizes": [
      "w45",
      "w185",
      "h632",
      "original"
    ],
    "still_sizes": [
      "w92",
      "w185",
      "w300",
      "original"
    ]
  },
...
}

Then, build the complete url of your poster (php):

$url = $config['images"]["secure_base_url"] . $config["images"]['poster_sizes"]["w500"] . $poster_path; // 500px wide image