JavScript อยู่ดีๆก็ไม่ทำงาน ดึงค่าจาก drop down list ใน html + php อะครับ มือใหม่ แนะนำที

echo "<div><input id='user_email2' class='mgm_register_field required' disabled='disabled' name='user_email' type='email' value='$show_country' /></div>";
echo '<div class="label_cert_form"><p class="cert"><label class="mgm_field_label" for="mgm_register_field_gender">Gym**</label></p><div>';

echo '<select class="mgm_register_field required" name="gym" onchange="findselected()">
<option value="s">.. Select Gym ..</option>';
while($menu=mysql_fetch_assoc($result)){
echo "<option value='$menu[Short_code_gym]'>$menu[Gym_member_is_NF]</option>";}
echo '<option value="o">Other...</option></select>';
echo "<div><input id='mgm_register_field_first_name' class='mgm_register_field required' disabled='disabled' name='gym_name' type='text' placeholder='Other' /></div>";

<script>
                                    function findselected(){
                                    var x = document.getElementById("gym").value;
                                    var y = x;
                                        if (x == 'o') {
                                            document.cert.gym_name.disabled=false;
                                        }                                      
                                        else{
                                            document.cert.gym_name.disabled=true;
                                            document.cert.gym_name.value= "";
                                        }
                                        if (y != 's'){
                                           if (y != 'o'){
                                           document.cert.g1.value= x.substring(0,1);
                                           document.cert.g2.value= x.substring(1,2);
                                           document.cert.g3.value= x.substring(2);
                                           }else { 
                                                  document.cert.g1.value= "";
                                                  document.cert.g2.value= "";
                                                  document.cert.g3.value= ""; }
                                       }
                                    }
</script>