query sql in php

$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";
    }
} else {
    echo "0 results";
}
$conn->close();

<?php
// Open database connection
$mysqli = new mysqli("localhost","my_user","my_password","my_db");

// Check connection
if ($mysqli -> connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli -> connect_error;
  exit();
  }

// Perform a query 
if ($result = $mysqli -> query("SELECT * FROM Persons")) {
  echo "Returned rows are: " . $result -> num_rows;
  // Free the result set
  $result -> free_result();
}

// Close database connection
$mysqli -> close();
?>

<?php 
// MySqli connection
$mysqli = new mysqli("localhost", "my_username", "my_password", "my_database");

// Errors
if($mysqli->connect_error) {
  echo "<b>Failed to connect to MySQL: </b>" . $mysqli->connect_error;
}

// Query
$selectQuery = $mysqli->prepare("SELECT * FROM my_table");
$selectQuery->execute();
$selectQueryResult = $selectQuery->get_result();

// Loop 
while($selectQueryRow = $selectQueryResult->fetch_array()) {
  echo $selectQueryRow['my_column'];
}

// Example
$selectUsername = $mysqli->prepare("SELECT * FROM users");
$selectUsername->execute();
$selectUsernameResult = $selectUsername->get_result();
while($selectUsernameRow = $selectUsernameResult->fetch_array()) {
  echo $selectUsernameRow['username'];
}