If you don't want to save strings in clear text, there are new php functions (php >= 5.3.0) that can be of help; openssl_encrypt() and openssl_decrypt().
$string = "This is a readable string."
$password = "p@ssword";
$method = "aes-256-cbc";
$encrypted = openssl_encrypt($string, $method, $password);
echo "$string => $encrypted";
// Outputs: This is a readable string. => OeOiTWcgIPC1xIZaDJ3XTEaY/D4m1sQmxgPbzjxxlRA=
$decrypted = openssl_decrypt($encrypted, $method, $password);
echo "$encrypted => $decrypted";
// Outputs: OeOiTWcgIPC1xIZaDJ3XTEaY/D4m1sQmxgPbzjxxlRA= => This is a readable string.
According to http://stackoverflow.com/questions/1391132/two-way-encryption-in-php, these are possible values for encryption methods.
aes-128-cbc, aes-128-cfb, aes-128-cfb1, aes-128-cfb8, aes-128-ecb, aes-128-ofb, aes-192-cbc, aes-192-cfb, aes-192-cfb1, aes-192-cfb8, aes-192-ecb, aes-192-ofb, aes-256-cbc, aes-256-cfb, aes-256-cfb1, aes-256-cfb8, aes-256-ecb, aes-256-ofb, bf-cbc, bf-cfb, bf-ecb, bf-ofb, camellia-128-cbc, camellia-128-cfb, camellia-128-cfb1, camellia-128-cfb8, camellia-128-ecb, camellia-128-ofb, camellia-192-cbc, camellia-192-cfb, camellia-192-cfb1, camellia-192-cfb8, camellia-192-ecb, camellia-192-ofb, camellia-256-cbc, camellia-256-cfb, camellia-256-cfb1, camellia-256-cfb8, camellia-256-ecb, camellia-256-ofb, cast5-cbc, cast5-cfb, cast5-ecb, cast5-ofb, des-cbc, des-cfb, des-cfb1, des-cfb8, des-ecb, des-ede, des-ede-cbc, des-ede-cfb, des-ede-ofb, des-ede3, des-ede3-cbc, des-ede3-cfb, des-ede3-cfb1, des-ede3-cfb8, des-ede3-ofb, des-ofb, desx-cbc, rc2-40-cbc, rc2-64-cbc, rc2-cbc, rc2-cfb, rc2-ecb, rc2-ofb, rc4, rc4-40, seed-cbc, seed-cfb, seed-ecb, seed-ofb
If you don't know what to choose, try "aes-256-cbc". AES is said to be used by U.S. government.
For alternatives that can be used in older PHP versions, check out, http://us2.php.net/manual/en/refs.crypto.php.
Don't use two-way encryptions on passwords. They should be encrypted with one-way hash functions.