// Assume that we have established a database connection using mysqli $email = $_POST['email']; // retrieve email from user input $query = "SELECT id FROM users WHERE email = '$email'"; $result = mysqli_query($connection, $query); if (mysqli_num_rows($result) == 1) { $row = mysqli_fetch_assoc($result); $id = $row['id']; // retrieve id from query result // perform authentication and other user-specific actions }In this example, we use the SQL query to search for a row in the database table 'users' that matches the given email address. Once we have the query result, we retrieve the id from the row using the mysqli_fetch_assoc function and assign it to a variable for further use. The getIdByEmail function is not a built-in PHP function; it is likely to be part of a package or library for user authentication and management. Some popular examples are: - Laravel Passport: a package for OAuth2 authentication that provides methods for managing user authorization with email and password. - Symfony Security: a component for authentication and authorization that provides a user provider interface with a method for retrieving users by email address. - Zend\Authentication: a component for authentication that provides an adapter interface for verifying user credentials with an email and password combination.