how to only update select fields on a database - php

Question

I have made a users account page where they can update their information.
For my first issue I will be using birthday as an example (it uses three dropdown bars which are saved as the variables $dobday $dobmonth and $dobyear) if a user only inputs one of the variable's - eg $dobdayand then they press 'update' the php will save that to the database, however this is then saving an incomplete date of birth, i have thought about adding a 'requirerd' function on the form, however i want it to be optional if the user only wants to fill out one piece of information. Is there any way I can resolve this by an if statement or another function that I am un aware of?
Secondly, if I enter a number in the phone number field and leave the rest of the fields blank on the form, when the database is updated, the other fields that were not filled in but had something saved in them on the database are wiped.
I am still learning web design so forgive me for my poor code. Thank you very much for your help and time :)
HTML form:
<form class="containerformAccounts" method="post" action="accountpageForm.php" >
<div class="container">
<h1>Edit your account</h1>
<hr>
<h3>Enter your telephone number</h3>
<label for="mobNumber"><b><i>Phone Number:</i></b></label>
<input type="mobNumber" maxlength="11" name="mobNumber" id="mobNumber" placeholder="Enter phone number"> <!--ommiting 'required' as this part is optional-->
</br>
<h3>Enter your address</h3>
<label for ="addressLine"><b><i>Address Line:</i></b></label>
<input type="addressLine" name="addressLine" id="addressLine" placeholder="Enter your address line">
<label for ="townCity"><b><i>Town/City:</i></b></label>
<input type="townCity" name="townCity" id="townCity" placeholder="Enter your town/city">
<label for ="postcode"><b><i>Postcode:</i></b></label>
<input type="postcode" name="postcode" id="postcode" placeholder="Enter your Postcode">
<label for ="country"><b><i>Country:</i></b></label>
<input type="country" name="country" id="country" placeholder="Enter your country">
</br>
<h3>Enter your birthdate</h3>
<label for ="dob-day"><b><i>Birthday:</i></b></label>
<select name="dobday" id="dobday">
<option value="Day">Day</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20</option>
<option value="21">21</option>
<option value="22">22</option>
<option value="23">23</option>
<option value="24">24</option>
<option value="25">25</option>
<option value="26">26</option>
<option value="27">27</option>
<option value="28">28</option>
<option value="29">29</option>
<option value="30">30</option>
<option value="31">31</option>
</select>
<select name="dobmonth" id="dobmonth">
<option value="Month">Month</option>
<option value="January">January</option>
<option value="Feburary">Feburary</option>
<option value="March">March</option>
<option value="April">April</option>
<option value="May">May</option>
<option value="June">June</option>
<option value="July">July</option>
<option value="August">August</option>
<option value="September">September</option>
<option value="October">October</option>
<option value="November">November</option>
<option value="December">December</option>
</select>
<select name="dobyear" id="dobyear">
<option value="Year">Year</option>
<option value="1950">1950</option>
<option value="1951">1951</option>
<option value="1952">1952</option>
<option value="1953">1953</option>
<option value="1954">1954</option>
<option value="1955">1955</option>
<option value="1956">1956</option>
<option value="1957">1957</option>
<option value="1958">1958</option>
<option value="1959">1959</option>
<option value="1960">1960</option>
<option value="1961">1961</option>
<option value="1962">1962</option>
<option value="1963">1963</option>
<option value="1964">1964</option>
<option value="1965">1965</option>
<option value="1966">1966</option>
<option value="1967">1967</option>
<option value="1968">1968</option>
<option value="1969">1969</option>
<option value="1970">1970</option>
<option value="1971">1971</option>
<option value="1972">1972</option>
<option value="1973">1973</option>
<option value="1974">1974</option>
<option value="1975">1975</option>
<option value="1976">1976</option>
<option value="1977">1977</option>
<option value="1978">1978</option>
<option value="1979">1979</option>
<option value="1980">1980</option>
<option value="1981">1981</option>
<option value="1982">1982</option>
<option value="1983">1983</option>
<option value="1984">1984</option>
<option value="1985">1985</option>
<option value="1986">1986</option>
<option value="1987">1987</option>
<option value="1988">1988</option>
<option value="1989">1989</option>
<option value="1990">1990</option>
<option value="1991">1991</option>
<option value="1992">1992</option>
<option value="1993">1993</option>
<option value="1994">1994</option>
<option value="1995">1995</option>
<option value="1996">1996</option>
<option value="1997">1997</option>
<option value="1998">1998</option>
<option value="1999">1999</option>
<option value="2000">2000</option>
<option value="2001">2001</option>
<option value="2002">2002</option>
<option value="2003">2003</option>
<option value="2004">2004</option>
<option value="2005">2005</option>
<option value="2006">2006</option>
<option value="2007">2007</option>
<option value="2008">2008</option>
<option value="2009">2009</option>
<option value="2010">2010</option>
<option value="2011">2011</option>
<option value="2012">2012</option>
<option value="2013">2013</option>
<option value="2014">2014</option>
<option value="2015">2015</option>
<option value="2016">2016</option>
<option value="2017">2017</option>
<option value="2018">2018</option>
<option value="2019">2019</option>
<option value="2020">2020</option>
</select>
</p>
<hr>
<h3>Change your password</h3>
</p><a href='passwordChange.php'>Change your password</a> </br>
</br>
<input type='submit' name='submit' value= 'Update'>
</hr>
</hr>
</div>
PHP:
<?php
session_start();//creating session
$user=$_SESSION['firstName'];
if (isset($_POST['submit'])){
$connectDB = mysqli_connect("localhost","root","") or die("cant connect"); //proving the database connection details and saving it as a variable
mysqli_select_db($connectDB, "registration"); //table name
// BIRTHDAY
$updateDBvalues=array(); //creating array to be used in query
$updateArray=array(); //creating array
$dobday=$_POST['dobday']; //importing variables
$dobmonth=$_POST['dobmonth'];//importing variables
$dobyear=$_POST['dobyear'];//importing variables
//ADDRESS
$addressLine=$_POST['addressLine'];//importing variables
$townCity=$_POST['townCity'];//importing variables
$postcode=$_POST['postcode'];//importing variables
$country=$_POST['country'];//importing variables
//NUMBER
$mobNumber=$_POST['mobNumber'];//importing variables
//NUMBER BELOW
if(!empty($mobNumber))
$updateArray[]="mobNumber='".$mobNumber."'";
//ADDRESS BELOW
if(!empty($addressLine))//if the variables is NOT empty
$updateArray[]="addressLine='".$addressLine."'";//add the variable to the array
if(!empty($townCity))//if the variables is NOT empty
$updateArray[]="townCity='".$townCity."'";//add the variable to the array
if(!empty($postcode))//if the variables is NOT empty
$updateArray[]="postcode='".$postcode."'";//add the variable to the array
if(!empty($country))//if the variables is NOT empty
$updateArray[]="country='".$country."'";//add the variable to the array
//BIRTHDAY BELOW
if(!empty($dobday))//if the variables is NOT empty
$updateArray[]="dobday='".$dobday."'";//add the variable to the array
if(!empty($dobmonth))//if the variables is NOT empty
$updateArray[]="dobmonth='".$dobmonth."'";//add the variable to the array
if(!empty($dobyear))//if the variables is NOT empty
$updateArray[]="dobyear='".$dobyear."'";//add the variable to the array
$updateDBvalues=$updateArray;
$updateDBvalues_imploded=implode(',',$updateDBvalues); // takes the array and makes one string
if(!empty($updateDBvalues)){ //if the array is NOT empty
$query="UPDATE users SET $updateDBvalues_imploded WHERE firstName='$user'";//sql query to save entries into table
$connQuery=mysqli_query($connectDB,$query);//querys database
die("Succesfully updated, return to <a href='accountPage.php'>Accounts page</a>");//successful query, provides link for user to return to account page
}else{
die ("Unable to save new infomation to the database, please try again. <a href='accountPage.php'>Accounts page</a>");//kills program, outputting that the query was unsuccessful, provides link for user to return to account page
}
}
?>

Answer 1

Well, you don't want to nullify your previously save data in fields. For that you can use COALESCE function in MySQL. It returns the first not null value in the parameters.
For example, you can set fields this way:
$updateArray[]="dobyear= coalesce('".$dobyear."', dobyear)"
But what about the situations where you really want to nullify the field. The better approach will be to populate the form with data in the database. Now, you can simply save the complete form with changes by the user.