Multiple search form using functions and if else

Hi Im creating a multiple search form using PHP,HTML,SQL with the use of functions, for example I have 3 search fields Firstname, lastname and email. I would let the user input from any of those, therefore i would be needing the if else statement, but to be able to satisfy all conditions it would take a lot of if else, so i think of using a function to output the table and place it inside the if else after the query on the database. But it seems that it could not be able to search in the database if I do it like this it outputs "0 results", but if i remove the function and place it on the end of my script I am able to search in the db but it could not detect my else condition which is "You have not yet entered any values"

    function checkres()
    {
    //Get query on the database
        $result = mysqli_query($conn, $sql);

        //Check results
        if (mysqli_num_rows($result) > 0)
        {
        //Headers
            echo "<table border='1' style='width:100%'>";
        echo "<tr>";
            echo "<th>Image ID</th>"; 
        echo "<th>Lastname</th>";
        echo "<th>Firstname</th>";
        echo "<th>Email</th>";
        echo "<th>PhoneNumber</th>";
        echo "</tr>";

      //output data of each row
          while($row = mysqli_fetch_assoc($result))
            {
            echo "<tr>";
                      echo "<td>".$row['ID']."</td>";
                      echo "<td>".$row['LastName']."</td>";
                      echo "<td>".$row['FirstName']."</td>";
                      echo "<td>".$row['Email']."</td>";
                      echo "<td>".$row['PhoneNumber']."</td>";
                    echo "</tr>";

            }
                  echo "</table>";
        } else {
            echo "0 results";
        }
    }

   if (!empty($sfname) && empty($slname) && empty($semail) )
{
     $sql = "select * from Userlist where FirstName LIKE '%". $sfname  ."%'" ;

checkres();

}

 else  if (!empty($sfname) && !empty($slname) && empty($semail))

{
  $sql = "select * from Userlist where FirstName LIKE '%". $sfname  ."%' AND LastName LIKE '%". %slname. "%'";

checkres();

}

else
{
   echo "You have not yet entered any values ";
}

mysqli_close($conn);

?>