<!-- index.html-->
<form action="process.php" method="post">

  <input type="text" name="first_name" placeholder="First Name" /><br/>
   <input type="text" name="last_name" placeholder="Last Name" /><br/>

      <button type="submit" name="submit"></button>
</form>

//database.php
$dbhost = "localhost";
$dbuser = "root";
$dbpass = "xxxx";
$dberror1 = "Could not connect to the database!";
$dberror2 = "Could not find selected table!";

// Connection to the database, Already tried this with echo statement and works
$conn  = mysqli_connect($dbhost, $dbuser, $dbpass) or die ($dberror1);

// Selecting the database to connect to
$select_db = mysqli_select_db($conn, 'mainbase') or die ($dberror2);

//process.php
<?php include 'database.php';

 if(isset($_POST['submit'])) {
// Creating variables to store form values
$first_name= $_POST['first_name'];
$last_name=$_POST['last_name'];


//Executing the query
mysqli_query($conn, " INSERT INTO 'candidates'('first_name', 'last_name') //Values in 'candidates' table on phpmyadmin
VALUES ('$first_name','$last_name')");/*variables from above*/
}