how to write query in php code example

Example 1: 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();

Example 2: query php

<?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'];
}