Why Does `mysqli_stmt::bind_result()` Throw a 'Number of Bind Variables Mismatch' Error in PHP?

mysqli_stmt::bind_result(): Number of Bind Variables Mismatch

When attempting to implement a PHP login form using a prepared statement, users commonly encounter the error "mysqli_stmt::bind_result(): Number of bind variables doesn't match number of fields in prepared statement." This error signifies a discrepancy between the number of bind variables used to store query results and the actual number of fields returned by the underlying SQL statement.

Diagnostic Code

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

Correction

The code above contains a mistake in its field bindings. The bind_result() method should reflect the actual fields returned by the SELECT statement. In this scenario, the query returns two fields: username and password. Therefore, the correct code is:

$stmt->bind_result($username, $password); // Corrected field binding

Further Tips

• Ensure that the SQL statement is correct. It should use proper syntax and the correct number of question marks (?) to represent bind variables.
• The data types of bind variables and result fields must match. In this case, both $username and $password should be strings.
• Always use prepared statements to prevent SQL injection vulnerabilities.

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