Undefined variable in checkbox array [duplicate]

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• PHP: “Notice: Undefined variable”, “Notice: Undefined index”, and “Notice: Undefined offset” 26 answers

I'm working on selecting multiple checkboxes and store it to my database but when I call the name of the checkboxes which is array "living_whom[]" and store it to my variable in php the error say that "Undefined index: living_whom". I can't find what is problem. Here's the code:

             <div class="row">
                <div class="col-md-12">
                  <div class="form-group">
                    <label>Living with whom</span></label>
                    <div class="row">
                      <div class="col-md-2">
                        <label class="checkbox-inline">Parents
                          <input type="checkbox" name="living_whom[]" value="Parents">
                          <span class="checkmark"></span>
                        </label>
                      </div>
                      <div class="col-md-3">
                        <label class="checkbox-inline">Brothers & Sisters
                          <input type="checkbox" name="living_whom[]" value="Brothers & Sister">
                          <span class="checkmark"></span>
                        </label>
                      </div>
                      <div class="col-md-2">
                        <label class="checkbox-inline">Grandparents
                          <input type="checkbox" name="living_whom[]" value="Grandparents">
                          <span class="checkmark"></span>
                        </label>
                      </div>
                      <div class="col-md-2">
                        <label class="checkbox-inline">Other Relatives
                          <input type="checkbox" name="living_whom[]" value="Other Relatives">
                          <span class="checkmark"></span>
                        </label>
                      </div>
                      <div class="col-md-2">
                        <label class="checkbox-inline">Friends
                          <input type="checkbox" name="living_whom[]" value="Friends">
                          <span class="checkmark"></span>
                        </label>
                      </div>
                    </div>
                  </div>
                </div>
              </div>

and when I get the name of the checkboxes in php:

 $living_whomArr = $_POST["living_whom"];

Thank you for the help!