What potential pitfalls should be considered when implementing password encryption in PHP for an existing user base?

When implementing password encryption in PHP for an existing user base, one potential pitfall to consider is ensuring that the existing passwords are securely migrated to the new encryption method without losing any user data. It is important to handle password hashing and verification correctly to prevent security vulnerabilities. Additionally, proper error handling and testing should be conducted to ensure a smooth transition for users.

// Example code snippet for migrating existing passwords to new encryption method

// Retrieve existing passwords from database
$existingPasswords = // Query to retrieve existing passwords

// Loop through each existing password and rehash using new encryption method
foreach ($existingPasswords as $password) {
    $newHashedPassword = password_hash($password, PASSWORD_DEFAULT);
    
    // Update database with new hashed password
    // Query to update database with new hashed password
}