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<?php
// Database connection details
$host = 'localhost';
$dbname = 'your_database';
$username = 'your_username';
$password = 'your_password';

// Create a new PDO instance
try {
    $pdo = new PDO("mysql:host=$host;dbname=$dbname;charset=utf8", $username, $password);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
    die("Could not connect to the database: " . $e->getMessage());
}

// Query to retrieve the source_id meta key and corresponding user_id
$query = "
    SELECT um.user_id, um.meta_value AS source_id
    FROM user_meta um
    WHERE um.meta_key = 'source_id'
";

try {
    // Execute the query
    $stmt = $pdo->query($query);
    
    // Fetch all results
    $results = $stmt->fetchAll(PDO::FETCH_ASSOC);

    // Prepare the update query
    $updateQuery = "
        UPDATE target_table
        SET user_id = :user_id
        WHERE source_id = :source_id
    ";
    $updateStmt = $pdo->prepare($updateQuery);

    // Loop through the results and update the target table
    foreach ($results as $row) {
        $updateStmt->execute([
            ':user_id' => $row['user_id'],
            ':source_id' => $row['source_id']
        ]);
    }

    echo "Update completed successfully.";
} catch (PDOException $e) {
    echo "An error occurred: " . $e->getMessage();
}
?>