AddCar.php
<?php
if ($_SERVER['REQUEST_METHOD'] === 'POST') {

    $name = $_POST['Name'];
    $number = $_POST['Number'];
    $connection = mysqli_connect('localhost', 'root', 'root', 'parking');
    $pyk = mysqli_query($connection, "INSERT INTO cars (`carName`, `carNumber`) VALUES ('$name', '$number')");
    mysqli_close($connection);

    echo "<h1>Машина добавлена!</h1><br>
        <a href = 'index.php'>Главное меню</a>" . $number;
} else {

    echo '
        <form action="/addCar.php" method="POST">
            <label for="Name">Название:</label><br>
            <input type="text" id="Name" name="Name"><br>
            <label for="Number">Номер машины:</label><br>
            <input type="text" id="Number" name="Number"><br><br>
            <input type="submit" value="Submit">
        </form>
        ';
}
Form.php
<meta charset="utf-8">
<?php
    $sql_server = 'localhost';
    $sql_user = 'root';
    $sql_pwd = 'root';
    $sql_db = 'parking';
    $connect = mysqli_connect($sql_server, $sql_user, $sql_pwd, $sql_db);
    if (!$connect) {
        exit("Ошибка " . mysqli_connect_error());
    }
    $res = mysqli_query($connect, "SELECT * FROM places WHERE Reserved = 0;");
    $parkingArr = mysqli_fetch_array($res);
    if(sizeof($parkingArr) == 0)
    {
        echo "Свободных мест нет";
    }
    else
    {
        echo '<h1>Парковка</h1>
              <form action="index.php" method = "GET">
              <p>Номер места
              <select>';
        foreach ($res as $row)
        {
            echo "<option value='" . $row['ID'] . "'>" . $row['ID'] . "</option>";
        }
        echo '</select></p>
        <p>Название машины <input type="text" name = "CarName"></p>
        <p>Номер машины <input type="text" name = "CarNum"></p>
        <p><input type="submit" value = "Занять место"></p>
        </form>';
    }
?>
Index.php
<?php

include 'show.php';

echo '
  <a href="\addCar.php"><button>Добавить машину</button></a>
  <a href="\takePlace.php"><button>Занять парковочное место</button></a>
  <a href="\removePlace.php"><button>Освободить парковочное место</button></a><br><br>

  <form action="index.php" mothod="GET">
  <button name="button" value="1">Показать парковочные места</button>
  <button name="button" value="2">Показать машины</button>
  ';

if ($_GET['button'] == 1)
    show_park();
elseif ($_GET['button'] == 2)
    show_cars();
removePlace.php
<meta charset="utf-8">
<?php

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $placeID = $_POST['placeID'];

    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');

    $stmt1 = mysqli_query($connect, "UPDATE places SET `Reserved`=0 WHERE ID = '$placeID'");
    $stmt1 = mysqli_query($connect, "DELETE FROM `parking` WHERE PlaceID = '$placeID'");

    mysqli_close($connect);
    echo "<h1>Место свободно!</h1><br>
    <a href = 'index.php'>Главное меню</a>";
} else {
    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');

    $resPlaces = mysqli_query($connect, "SELECT * FROM places WHERE Reserved = 1;");

    $parkingArr = mysqli_fetch_array($resPlaces);
    if (sizeof($parkingArr) == 0) {
        echo "Все места свободны";
    } else {
        echo "<h1>Парковка</h1>
              <form action='removePlace.php' method = 'POST'>
              <p>Номер места
              <select name='placeID'>";
        foreach ($resPlaces as $row) {
            echo "<option value= " . $row['ID'] . " >" . $row['ID'] . "</option>";
        }
        echo "</select>
        <p><input type='submit' value = 'Освободить место'></p>
        </form>";
    }
    mysqli_close($connect);
}
?>
show.php
<?php

function show_park()
{
    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');
    $resPlaces = mysqli_query($connect, "SELECT * FROM parking");
    echo '<table><tr><th>ID</th><th>Парковочное место</th><th>Машина</th></tr>';
        while($row = mysqli_fetch_array($resPlaces))
        {
            echo '<tr><td>'.$row['ID'].'</td><td>'.$row['PlaceID']."</td><td>".$row['CarID'].'</td></tr>';
        }
        echo '</table>';
}

function show_cars()
{
    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');
    $resCars = mysqli_query($connect, "SELECT * FROM cars");
    echo '<table><tr><th>ID</th><th>Название машины</th><th>Номер</th></tr>';
        while($row = mysqli_fetch_array($resCars))
        {
            echo '<tr><td>'.$row['ID'].'</td><td>'.$row['carName']."</td><td>".$row['carNumber'].'</td></tr>';
        }
        echo '</table>';
}
takePlace.php
<meta charset="utf-8">
<?php

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $placeID = $_POST['placeID'];
    $carID = $_POST['carID'];

    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');

    $stmt = mysqli_prepare($connect, "INSERT INTO parking (`PlaceID`, `CarID`) VALUES (?, ?)");
    $stmt->bind_param("si", $placeID, $carID);
    $stmt->execute();
    mysqli_stmt_close($stmt);

    $stmt1 = mysqli_query($connect, "UPDATE places SET `Reserved`=1 WHERE ID = '$placeID'");

    mysqli_close($connect);
    echo "<h1>Место ваше!</h1><br>
    <a href = 'index.php'>Главное меню</a>";
} else {
    $connect = mysqli_connect('localhost', 'root', 'root', 'parking');

    $resPlaces = mysqli_query($connect, "SELECT * FROM places WHERE Reserved = 0;");
    $resCars = mysqli_query($connect, "SELECT * FROM cars");

    $parkingArr = mysqli_fetch_array($resPlaces);
    if (sizeof($parkingArr) == 0) {
        echo "Свободных мест нет";
    } else {
        echo "<h1>Парковка</h1>
              <form action='takePlace.php' method = 'POST'>
              <p>Номер места
              <select name='placeID'>";
        foreach ($resPlaces as $row) {
            echo "<option value= ". $row['ID'] ." >" . $row['ID'] . "</option>";
        }
        echo "</select>
              <select name='carID'>";
        foreach ($resCars as $row) {
            echo "<option value= ". $row['ID'] ." >" . $row['carName'] . ' | ' . $row['carNumber'] . "</option>";
        }
        echo '</select></p>

        <p><input type="submit" value = "Занять место"></p>
        </form>';
    }
    mysqli_close($connect);
}
?>