PHP/MySQL : error to save data when data all checkbox is uncheck

Question

I'm new in php programming. I have one question here. Currently i have 12 checkbox created using array implode. when i check for example, one checkbox, the data will save in database but when i uncheck all the checkbox(no single checkbox was checked), there is an error occur. below is the error:

Notice: Undefined index: transportMode in C:\xampp\htdocs\tggd\page\patient-health-care\edit-patient-health_2.php on line 11

Warning: implode(): Invalid arguments passed in C:\xampp\htdocs\tggd\page\patient-health-care\edit-patient-health_2.php on line 11

below is the current code:

edit-patient-health2.php

        <?php                            
        $checked_arr = array();

        $checked_arr = explode(",",$transportMode);
        
        $transportMode = array("Transported","Transport Refused/declines","Decrease on examination","Operational Standby","Sporting/special event standby","Unable to locate patient","Assist another ambulance","Transport retrieval team","SIDS","S/D Assault","Single journey","Return journey","Escort police","Nurse","MO");
        foreach($transportMode as $transportMode1){
            $checked = "";
            if(in_array($transportMode1,$checked_arr)){
                $checked = "checked";
            }
            echo 
                '<input type="checkbox" name="transportMode[]" class="form-check-input; larger" style="margin-right: 20px; margin-left: 50px;" value="'.$transportMode1.'" '.$checked.'> '.$transportMode1.'  <br/>';
                
        }
    ?>

edit-patient-health_2.php

   $transportMode=implode(",",$_POST["transportMode"]); //line 11

can someone help me? thank you