$stmt variable contains null using prepare function [duplicate] - php

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$stmt variable is null means it doesn't contain my query even though connection($con) to the database is created.
Fatal error: Uncaught Error: Call to a member function bind_param() on boolean in C:\xampp\htdocs\WebServersAndroid\includes\DbOperations.php:24 Stack trace: #0 C:\xampp\htdocs\WebServersAndroid\v1\registerUser.php(12): DbOperation->createUser('syed', '1997') #1 {main} thrown in C:\xampp\htdocs\WebServersAndroid\includes\DbOperations.php on line 24
error occurs in bind_param maybe because there is no query to store data to. Please tell me what is the problem and why $stmt is null. Thank you
class DbOperation{
private $con;
function __construct(){
require_once dirname(__FILE__).'/DbConnect.php';
$db = new DbConnect();
$this->con = $db->connect();
}
function createUser($name,$usn){
$query = "INSERT INTO 'test_col'('name','usn') VALUES (?,?); ";
$stmt = $this->con->prepare($query);
echo $stmt;
if($stmt != null)
{
$stmt->bind_param("ss",$name,$usn);
if($stmt -> execute())
{
return true;
}else{
return false;
}
}
else{
echo "in else";
}
}
}

Answer 1

I think this code generates a lot of errors.
$query = "INSERT INTO 'test_col'('name','usn') VALUES (?,?); ";
values (?, ?) expects 2 values to be bind later.
While this code
`$stmt->bind_param("ss",$name,$usn);`
binds 3 parameters which trigger an error in binding.
Next, the use of single/double quotation mark is interpreted by MySQL as String literal as stated in documantation
As a result, 'test_col'('name','usn') causes error, you should use ` or just leave the name of table and columns instead of using single quotation mark (') which translate your table name, database name, and/or column name to string instead of reference.