Required all fields

Hello there, anyone can help me with my problem?

my problem is, i want to display a "please complete all the fields" when the user inputs incomplete data in the textboxes and i want to display it in the same form.

im new in php.

These are my codes

<?php
		
		$posapp = $_POST['txtapplied']; 
		$fname = $_POST['txtfname']; 
		$mname = $_POST['txtmname']; 
		$lname = $_POST['txtlname']; 
		$month=$_POST['txtmonth'];
		$day=$_POST['txtday'];
		$year=$_POST['txtyear'];
		$age=$_POST['txtage'];
		$eadd=$_POST['txtemail'];
		$homenumber=$_POST['txthnum'];
		$cpnumber=$_POST['txtcpnumber'];
		$address=$_POST['txtaddress'];
		$religion=$_POST['txtreligion'];
		$birthplace=$_POST['txtbplace'];
		$height=$_POST['txtheight'];
		$weight=$_POST['txtweight'];
		$civilstatus=$_POST['txtcstatus'];
		$college=$_POST['txtcollege'];
		$course=$_POST['txtcourse'];
		$yeargrad=$_POST['txtyeargrad'];
		
		$db=mysql_connect('localhost','root','') or die ('Cannot connect to MYSQL!');
		@mysql_select_db('dbriomra') or die('Cannot connect to database');

		
		$query="INSERT INTO onlineapplication (positionapplied, firstname, middlename, lastname, month, day, year, age, emailaddress, homenumber, cellphone, address, religion, birthplace, height, weight, civilstatus, college, course, yeargrad) VALUES ('$posapp', '$fname', '$mname', '$lname', '$month', '$day', '$year', '$age', '$eadd', '$homenumber', '$cpnumber', '$address', '$religion', '$birthplace', '$height', '$weight', '$civilstatus', '$college', '$course', '$yeargrad')";

       
  //$result = mysql_query($query); 
  
  if(mysql_query($query))
        {
         header('Location: oapp_finished.php');
        }
        else
        {
         echo "<br> Problem Loading";
        }
     
       mysql_close($db);
?>