How can unique constraints in a MySQL table be utilized to prevent users from voting multiple times in a PHP application?

To prevent users from voting multiple times in a PHP application using MySQL unique constraints, you can create a unique constraint on the user ID and the item being voted on. This will ensure that each user can only vote once for each item. If a user tries to vote again, MySQL will throw an error that can be caught in your PHP code to display a message to the user. 

// Establish a database connection
$connection = mysqli_connect("localhost", "username", "password", "database");

// Check if the user has already voted for the item
$user_id = 1; // Assuming the user ID is 1
$item_id = 5; // Assuming the item ID is 5
$query = "INSERT INTO votes (user_id, item_id) VALUES ($user_id, $item_id)";
if(mysqli_query($connection, $query)) {
    echo "Vote successful!";
} else {
    if(mysqli_errno($connection) == 1062) {
        echo "You have already voted for this item.";
    } else {
        echo "Error: " . mysqli_error($connection);
    }
}

// Close the database connection
mysqli_close($connection);

Keywords

PHP MySQL unique constraints prevent voting

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