The mysqli_error() function in PHP is used to retrieve the last error message from the MySQL database connection. It returns a string that describes the last error that occurred during a query execution. If no error occurred, it will return an empty string.

Syntax:

mysqli_error(connection);

• connection: The MySQLi connection object (returned by mysqli_connect()).

Example Usage:

1. Basic Example: This example demonstrates using mysqli_error() to capture and display errors that occur during a database query.

<?php
// Create connection
$conn = mysqli_connect("localhost", "username", "password", "database");

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Incorrect SQL query to trigger an error
$sql = "SELECT * FROM non_existing_table";
$result = mysqli_query($conn, $sql);

// Check for errors
if (!$result) {
    echo "Error: " . mysqli_error($conn); // Display MySQL error message
}

mysqli_close($conn);
?>

Explanation:

• In this example, the query is incorrect because it references a table that doesn't exist (non_existing_table).
• The mysqli_error() function captures and displays the error message from MySQL (e.g., "Table 'database.non_existing_table' doesn't exist").

2. With Prepared Statements: Here’s how you can use mysqli_error() when working with prepared statements.

<?php
// Create connection
$conn = mysqli_connect("localhost", "username", "password", "database");

// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}

// Prepare a query with an error in it
$query = "INSERT INTO users (name, email) VALUES (?, ?)";
$stmt = mysqli_prepare($conn, $query);

if (!$stmt) {
    echo "Prepared Statement Error: " . mysqli_error($conn); // Check if prepare failed
}

// Bind parameters and execute
$name = 'John Doe';
$email = 'john.doe@example.com';
mysqli_stmt_bind_param($stmt, 'ss', $name, $email);

if (!mysqli_stmt_execute($stmt)) {
    echo "Execute Error: " . mysqli_error($conn); // Check if execution failed
}

mysqli_stmt_close($stmt);
mysqli_close($conn);
?>

Explanation:

• The example uses prepared statements to insert a user into the users table.
• If any error occurs during the preparation or execution of the query, mysqli_error() captures and displays the MySQL error message.

Key Points:

• mysqli_error() helps in debugging by providing error messages from the MySQL server.
• It is often used after a query or prepared statement execution to check for errors and handle them accordingly.
• Always ensure to handle errors properly in a production environment, possibly using custom error handling mechanisms.