<form method="post" action="search_doctor.php" name="f1">
      <select required class="form-dropdown" name="area_name">
      <option value="" selected="selected" disabled hidden required>Меня
      интересует...</option>
      <option value="d1">Терапевт</option>
      <option value="d2">Стоматолог</option>
      <option value="d3">Окулист</option>
      <option value="d4">Хирург</option>
      </select>
      </div>
      <br>
      <section class="filter-wrapper">
      <div class="keyword-wrapper">
      <input type="text" id="keyword" autocomplete="off"
      placeholder="Введите название клиники" name="hospital_name"/>
      </div>
      <ul id="filter-select" class="filter-select no-value">
      </ul>
      </section>
      <br>

      <div class="form-group">
     <label class="label-color">Дата приема</label>
     <input type="date" name="cid" class="form-control timepicker"
     max="2979-12-31">
     </div>

     <div class="form-time">
    <label class="label-color">Время</label>
    <input type="time" value="09:00" class="form-control timepicker"
    name="time_from">
    </div>

   <div class="form-time">
    <label class="label-color">Время</label>
    <input type="time" value="18:00" class="form-control timepicker"
    name="time_to">
    </div>
    <br><br>
    <button type="submit" class="btn btn-primary btn-block"
    name="filter">Найти</button><br/>
    </div>
    </form>

<?php
    require 'connect.php';
    function addWhere($where, $add, $and = true) {
    if ($where) {
    if ($and) $where .= " AND $add";
    else $where .= " OR $add";
    }
    else $where = $add;
    return $where;
    }
    if (!empty($_POST["filter"])) {
    $where = "";
    if ($_POST["area_name"]) $where = addWhere($where, "`area` >=
    '".htmlspecialchars($_POST["area_name"]))."'";
    if ($_POST["hospital_name"]) $where = addWhere($where, "`hospital` <=
    '".htmlspecialchars($_POST["hospital_name"]))."'";
    if ($_POST["date"]) $where = ???
    if ($_POST["time"]) $where = ???
    $sql = 'SELECT employee_name, employee_photo, e-mail, phone,
    hospital_name
    FROM (
    SELECT employee, area, hospital, schedule
    FROM employee, area, hospital, schedule
    )
   AS i
   LEFT JOIN group_attribute_hospital AS inst ON hospital_number =
   employee_number
   ';
   $second='employee_photo_main=0 ORDER BY employee_id DESC';
   if ($where) $sql .= " WHERE $where AND ".$second;
   else $sql .= " WHERE ".$second;
   }
   echo '<h1>Получили такой SQL запрос:</h1>'.$sql;
   $stmt = $pdo->prepare($sql);
   $stmt->execute($ids);
   $no=$stmt->rowCount();
   echo '<h2>В результате найдено: '.$no.' строк </h2>';
   $conn->close();
   ?>