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

Question

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 Statement
$stmt->close();
}
if ($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.

Answer 1

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 );

Answer 2

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();