Syntax Error with PHP/MySQL Insert Statement

I have been trying to figure this out for two days and can't seem to find any syntax issues. When I run the following piece of PHP code in my page I get the error listed below. All fields in the insert statement are varchar in the database.

Also I have two other fields in my database, one is called ID and is an INT with auto increment. The other is a date field called created and is set to CURRENT_TIMESTAMP.

<?php
//set form variables
$first=$_POST['first'];
$last=$_POST['last'];
$username=$_POST['username'];
$password=$_POST['password'];
$state=$_POST['state'];
$active=$_POST['active'];
$message="User added successfully!";

//Check connection
if (mysqli_connect_errno()) {
      echo 'Connection to database failed:' .mysqli_connect_error();
      exit();
      }
//Insert record
if (isset($_POST['submit'])){
      $sql = "INSERT INTO authors ('first', 'last', 'uname', 'password', 'state', 'active') VALUES ('$first','$last','$username','$password','$state','$active');";
       if (mysqli_query($conn, $sql)) {
                echo $message;
            } else {
                echo "Error: " . $sql . "<br>" . mysqli_error($conn); //display error
      }
}
?>
****************
Error: INSERT INTO authors ('first', 'last', 'uname', 'password', 'state', 'active') VALUES ('John','Smith','jsmith','1234','CT','T');
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''first', 'last', 'uname', 'password', 'state', 'active') VALUES ('John','Smith',' at line 1