insert value of html select form into mysql database

you have to wrap your select tag into a form tag .

<form action='' method='post'>
<select name="myselectbox">
   <option name="myoption1" value="myoption1">myoption1</option>
   <option name="myoption2" value="myoption2">myoption2</option>
   <option name="myoption3" value="myoption3">myoption3</option>
   <option name="myoption4" value="myoption4">myoption4</option>
</select>
<input type='submit' value='submit'/>
</form>

once you submit the form, you will get the post variable as $_POST['myselectbox'] that could be appended into a mysql query as you have already did. but for a better way dont just append it like that but check the form is submitted and post variables are available or not before appending. eg:

if(!empty($_POST['myselectbox'])){
    /*.. do your query section... */
}