Submitting a multiple checkbox form to MySql with php

I have a form with multiple fields: Textinputs, Checkboxes, Radios.. and I want to submit it to a MySQL database. When I comment the checkboxes HTML, and the corresponding php code, everything is working fine, and everything is submitted and saved in the DB. If I try to submit the checkbox-form and I uncomment it, nothing get submitted, and clicking on the submit-button doesn't make any effect.

How can I submit the value of the checkbox-field to the MySQL-Database as a string, with the values separated with a semi-colon? For ex. if the checkbox fields are: Ab, Cd, De, Fg - and "Ab" and "De" are checked, the following string gets submitted: "Ab;Cd"

Here is a part of my HTML-form:

<div class="row">
                                <div class="col-sm-4">
                                    <fieldset class="form-group">
                                        <label for="plattform">Platform</label>
                                     <form id="formId">


                                            <input type="checkbox" name="check_list[]" value="Android">Android
                                            <input type="checkbox" name="check_list[]" value="iPhone">iPhone
                                            <input type="checkbox" name="check_list[]" value="iPad">iPad
                                            <input type="checkbox" name="check_list[]" value="Windows Phone">Windows Phone

                                        </form> 

                                        <!-- <input type="checkbox" name="check_list[]" value="Android">
                                        <input type="checkbox" name="check_list[]" value="iPhone">
                                        <input type="checkbox" name="check_list[]" value="iPad">
                                        <input type="checkbox" name="check_list[]" value="Windows Phone"> --> 

                                    </fieldset>
                                </div>
                                <div class="col-sm-4">
                                    <fieldset class="form-group">
                                        <label for="featured">Featured</label>
                                            <div>
                                                <input type="radio" name="featured" required>True</input>
                                            </div>
                                            <div>
                                                <input type="radio" name="featured" required checked>False</input> 
                                            </div>
                                    </fieldset>
                                </div>

here is a sample of my php-file: