How can the use of functions like md5() in PHP code impact the security and efficiency of the application?

Using functions like md5() in PHP code can impact security because they are considered weak hashing algorithms that are susceptible to brute force attacks. It is recommended to use stronger hashing algorithms like bcrypt or Argon2 for securely storing passwords. Additionally, md5() is not efficient for hashing large amounts of data due to its speed and simplicity, which can lead to performance issues in applications.

// Using bcrypt for secure password hashing
$password = "password123";
$hashed_password = password_hash($password, PASSWORD_BCRYPT);