Why Does `mysqli_stmt::bind_result()` Throw a 'Mismatched Bind Variables' Error?

mysqli_stmt::bind_result(): Mismatched Bind Variables vs. Prepared Statement Fields

Question:

When attempting to execute a prepared statement with mysqli_stmt, I encounter the error: "mysqli_stmt::bind_result(): Number of bind variables doesn't match number of fields in prepared statement." Why am I receiving this error?

Code Example:

$mysqli = new mysqli("localhost", "root", "", "security");

$stmt = $mysqli->prepare("SELECT username AND password FROM users WHERE username = ?");
$username = $_POST['name'];
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->bind_result($password, $username);
$stmt->fetch();

Answer:

The error you are encountering occurs when the number of variables specified in bind_result() does not match the number of columns returned by the prepared statement query. In your example, you are attempting to bind two variables to a query that returns only one column.

To resolve this issue, the query should be written in the correct format for the operation. The proper syntax for selecting multiple fields is to separate them by commas, not AND:

$stmt = $mysqli->prepare("SELECT username, password FROM users WHERE username = ?");

Once the query is corrected, the bind_result() method can be used to correctly bind the variables to the result:

$stmt->bind_result($username, $password);

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