// function to get user ID by email function getUserIDByEmail($email) { // connect to database $conn = new mysqli("localhost", "username", "password", "database"); // query to get user ID $sql = "SELECT id FROM users WHERE email = '$email'"; $result = $conn->query($sql); // if user exists, return ID if ($result->num_rows > 0) { $row = $result->fetch_assoc(); return $row['id']; } else { return 0; // user not found } }In this example, we define a function called getUserIDByEmail which takes an email address as a parameter. Inside the function, we connect to a MySQL database and execute a SELECT query to retrieve the user ID where the email address matches the input parameter. If a user is found, their ID is returned. Otherwise, a default value of 0 is returned indicating that the user was not found. Package Library: The package library for this function is PHP MySQLi Extension.