checkbox value edit and update using explode php

How to fix the problem Notice: Undefined variable: b in C:\wamp64\www\MyFolder\version1.1\fms\build\english\equipment.php on line 904

Warning: in_array() expects parameter 2 to be array, null given in C:\wamp64\www\MyFolder\version1.1\fms\build\english\equipment.php on line 904

<div class="form-group">    
                                <label class="custom-control custom-checkbox" style="margin-left: 10px; margin-right: 35px;">
                                    <input type="checkbox" name="equipment[]" value="Tv"  class="custom-control-input"
                                    <?php
                                        if (isset($_POST["equipment_id"])) {

                                            $id = $_POST['equipment_id'];
                                            $query=mysqli_query("SELECT * FROM equipment WHERE id='$id'");
                                            $row=mysqli_fetch_array($query);

                                            $a = $row["equipment"];
                                            $b = explode(",", $a);
                                            print_r($b);

                                        } 

                                        if (in_array("Tv", $b)) {
                                            echo "checked";
                                        }
                                     ?>
                                    >
                                        <span class="custom-control-indicator"></span>
                                        <img src="img/equipments/tv-icon.png" width="25px" height="25px">
                                        <span class="custom-control-description">TVs</span>
                                </label>
                                <label class="custom-control custom-checkbox" style="margin-right: 35px;">
                                        <input type="checkbox" name="equipment[]" value="Phone" class="custom-control-input">
                                        <span class="custom-control-indicator"></span>
                                        <img src="img/equipments/phone-icon.png" width="25px" height="25px">
                                        <span class="custom-control-description">Phone</span>
                                </label>
                                <label class="custom-control custom-checkbox" style="margin-right: 35px;">
                                        <input type="checkbox" name="equipment[]" value="Fan" class="custom-control-input">
                                        <span class="custom-control-indicator"></span>
                                        <img src="img/equipments/fan-icon.png" width="25px" height="25px">
                                        <span class="custom-control-description">Fan</span>
                                </label>
                                <label class="custom-control custom-checkbox" style="margin-right: 35px;">
                                        <input type="checkbox" name="equipment[]" value="Mirror" class="custom-control-input">
                                        <span class="custom-control-indicator"></span>
                                        <img src="img/equipments/mirror-icon.png" width="25px" height="25px">
                                        <span class="custom-control-description">Mirrors</span>
                                </label>    
                            </div>