Prepared statement not letting me call $mysqli->stmt_init()

I have done this before but am quite new to mysqli and prepared statements (as I'm sure you can see from this issue).

Where am I going wrong?

here is my connection function (part of the 'Connect' class)

public function site_db()
{
    // Connect to MySQL 
    $link = mysqli_connect(SITE_HOST, SITE_ID, SITE_PW, SITE_DB); 

    // Check for Errors 
    if(mysqli_connect_errno()) 
    { 
        //echo mysqli_connect_error(); //shouldnt show client specific error information. 
        die('Error connecting to mysql database please report.'); 
    }
}

Heres the function which is causing the error:

public function exists ($what, $who)
{

    $query = "SELECT * FROM users WHERE ? = ?";

    // Get instance of statement 
    $stmt = $mysqli->stmt_init();

    // Prepare query 
    if($stmt->prepare($query)) 
    { 
        // Bind Parameters 
        $stmt->bind_param("ss", $what, $who); 

        // Execute statement 
        $stmt->execute(); 

        // Bind result variables 
        $stmt->bind_result($result); 

        // Fetch Value 
        $stmt->fetch(); 

        // catch num_rows result as variable
        $username_result = $result->num_rows;

        // Close State开发者_运维知识库ment 
        $stmt->close();
    }

    if ($username_result != 0)
    {
        return true;
        echo 'true';
    }
    else
    {
        return false;
        echo 'false';
    }
}

the error I get:

PHP Fatal error: Call to a member function stmt_init() on a non-object in somefile.php on line X

It is referring to the line:

$stmt = $mysqli->stmt_init();

am I making a stupid error here? Howcome I can't call that?

EDIT//

NOTE: I didn't make this very clear, but these two functions are within different classes.

public function site_db()
{
    // Connect to MySQL 
    $mysqli = mysqli_connect(SITE_HOST, SITE_ID, SITE_PW, SITE_DB); 

    // Check for Errors 
    if(mysqli_connect_errno()) 
    { 
        //echo mysqli_connect_error(); //shouldnt show client specific error information. 
        die('Error connecting to mysql database please report.'); 
    }

     return $mysqli;
}


public function exists (Mysqli $mysqli, $what, $who)
{

    $query = "SELECT * FROM users WHERE ? = ?";

    // Get instance of statement 
    $stmt = $mysqli->stmt_init();

    // Prepare query 
    if($stmt->prepare($query)) 
    { 
        // Bind Parameters 
        $stmt->bind_param("ss", $what, $who); 

        // Execute statement 
        $stmt->execute(); 

        // Bind result variables 
        $stmt->bind_result($result); 

        // Fetch Value 
        $stmt->fetch(); 

        // catch num_rows result as variable
        $username_result = $result->num_rows;

        // Close Statement 
        $stmt->close();
    }

    if ($username_result != 0)
    {
        return true;
        echo 'true';
    }
    else
    {
        return false;
        echo 'false';
    }
}

How to use:

Instantiate first class that have site_db() method

$db = new Class();

Then instantiate second class that have exist() method

$query = new Class();

Then simply

$query->exist($db->site_db(), $what, $who );

it's because your $mysqli is not declared inside function exists(). Try a global $mysqli inside function exists() if your $mysqli is declared outside the function.

Or, probably better - make $mysql an new object inside your Connect class:

$this->mysqli = mysqli_connect(SITE_HOST, SITE_ID, SITE_PW, SITE_DB); 

and in your function exists()

$stmt = $this->mysqli->stmt_init();