What are the potential issues with using MD5 for key encryption in PHP, especially when working with DES encryption?

Using MD5 for key encryption in PHP can be problematic because MD5 is considered to be a weak hashing algorithm and is not suitable for encryption purposes. This can lead to security vulnerabilities and potential attacks on the encrypted data. To solve this issue, it is recommended to use stronger hashing algorithms such as SHA-256 or SHA-512 for key encryption when working with DES encryption.

$key = hash('sha256', 'secret_key');