To prevent SQL Injection in PHP

I hope everyone can give me some advice, thank youhttps://github.com/stwater20/php_defense

<?php

  //You must replace the value in the braces
  require("database.inc.php");
  $link = mysqli_connect($SERVER, $USERNAME, $PASSWORD) or die("Connect Failed");
  mysqli_query($link, "SET NAMES 'UTF8'");
  mysqli_select_db($link, $DATABASE) or die("Select Failed");

  // define variables and set to empty values
  $user_id = $email = $picture_url = "";

  $allowed_hosts = array('{your_url}');
  if (!isset($_SERVER['HTTP_HOST']) || !in_array($_SERVER['HTTP_HOST'], $allowed_hosts)) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 400 Bad Request');
  } else {
    if ($_SERVER["REQUEST_METHOD"] == "POST") {
      $user_id = test_input($_POST["user_id"]);
      $display_name = test_input($_POST["display_name"]);
      $picture_url = test_input($_POST["picture_url"]);
      $query_show = "SELECT * FROM {table} WHERE {columns} = '$user_id'";
      $result_show = mysqli_query($link, $query_show) or die();
      if (mysqli_num_rows($result_show) == 0) {
        $sql = "INSERT INTO {table} VALUES('$user_id','$display_name','$picture_url',0)";
        $result = mysqli_query($link, $sql) or die();
      } else {
        $target_row = mysqli_fetch_array($result_show);
        $count = $target_row[3] + 1;
        $sql = "UPDATE {table} SET {columns} = $count WHERE {columns} = '$user_id'";
        $result = mysqli_query($link, $sql) or die();
      }
    }
  }

  function test_input($data)
  {
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
  }

  echo $user_id . " " . $display_name . " " . $picture_url;