Why Does My MySQLi Prepared Statement Throw a 'Number of bind variables doesn't match number of fields' Error?

Mismatched Bind Variables in MySQLi Prepared Statement

When attempting to implement a login form using a prepared statement in PHP, some developers encounter the frustrating error: "mysqli_stmt::bind_result(): Number of bind variables doesn't match number of fields in prepared statement."

This error arises when the SQL query and the bind_result() function contain a mismatch in the number of fields being selected and assigned to variables. Let's examine how to address this issue based on the provided code.

In the provided code:

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

The issue lies in the SQL query. The "AND" keyword is used instead of a comma when selecting multiple fields. The correct syntax for selecting multiple fields is to separate them with a comma:

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

Additionally, the bind_result() function should match the number of selected fields. In this case, two fields are selected, so the bind_result() function should have two variables:

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

By correcting the select syntax and aligning the bind_result() function with the number of selected fields, the issue of mismatched bind variables will be resolved, allowing the login form to function correctly.

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