$db = mysqli_connect('localhost', '*******', '********', '***********');


      $user_check_query = "SELECT partnerCodeOwner1 FROM refer where  randomfield='$partnerCode'";
  $result = mysqli_query($db, $user_check_query);
  $row = mysqli_fetch_array($result, MYSQLI_ASSOC);



// --------------------HERE I'm trying to check for the same value in columns--------
//But incorrect! Syntax error?

    if ($row) { // compare columns But incorrect
    if ($row['username'] === ['partnerCodeOwner1']) {
      array_push($errors, "U can't invest to yourself");
    }




    if(empty($row['partnerCodeOwner1'])) {
        $query = "update refer
                    set partnerCodeOwner1= '" . $_SESSION['username'] . "'
                where randomfield='$partnerCode' ;";
    }else {
        $query = "update refer
                    set partnerCode= '" . $_SESSION['username'] . "'
                where randomfield='$partnerCode'  ";
    }

    mysqli_query($db, $query);
}
?>

ID      randomfield    username      partnerCode    partnerCodeOwner1
+-------+ ------------+-------------+-----------------+-----------+
|   1   | A45scdv     |  John       |  Not same       |    John   |
+-------+-------------+-------------+-----------------+-----------+
|   2   | das654d     | Alex        |  Alex           | Not same  |
+-------+-------------+-------------+-----------------+-----------+