These are two simple functions for 256-bit encryption/decryption using mcrypt.

<?php

/**
 * These are two simple functions I built for 256-bit encryption/decryption with mcrypt.
 * I've decided to use MCRYPT_RIJNDAEL_128 because it's AES-compliant, and MCRYPT_MODE_CBC.
 * (ECB mode is inadequate for many purposes because it does not use an IV.)
 *
 * This function stores a hash of the data to verify that the data was decrypted
 * successfully, but this could be easily removed if necessary.
 *
 * @link http://php.net/manual/en/book.mcrypt.php
 * @param string $decrypted
 * @param string $password
 * @param string $salt
 * @return boolean|string
 */
function encrypt ($decrypted, $password, $salt = '!kQm*fF3pXe1Kbm%9')
{
    // Build a 256-bit $key which is a SHA256 hash of $salt and $password.
    $key = hash('SHA256', $salt . $password, true);

    // Build $iv and $iv_base64. We use a block size of 128 bits (AES compliant) and CBC mode. (Note: ECB mode is inadequate as IV is not used.)
    srand();

    $iv = mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC), MCRYPT_RAND);

    if (strlen($iv_base64 = rtrim(base64_encode($iv), '=')) != 22)
        return false;
        // Encrypt $decrypted and an MD5 of $decrypted using $key. MD5 is fine to use here because it's just to verify successful decryption.

    $encrypted = base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $decrypted . md5($decrypted), MCRYPT_MODE_CBC, $iv));

    // We're done!
    return $iv_base64 . $encrypted;
}

/**
 * Decrypt the stored hash value.
 *
 * @param string $encrypted
 * @param string $password
 * @param string $salt
 * @return boolean|string
 */
function decrypt ($encrypted, $password, $salt = '!kQm*fF3pXe1Kbm%9')
{
    // Build a 256-bit $key which is a SHA256 hash of $salt and $password.
    $key = hash('SHA256', $salt . $password, true);

    // Retrieve $iv which is the first 22 characters plus ==, base64_decoded.
    $iv = base64_decode(substr($encrypted, 0, 22) . '==');

    // Remove $iv from $encrypted.
    $encrypted = substr($encrypted, 22);

    // Decrypt the data. rtrim won't corrupt the data because the last 32 characters are the md5 hash; thus any \0 character has to be padding.
    $decrypted = rtrim(mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, base64_decode($encrypted), MCRYPT_MODE_CBC, $iv), "\0\4");

    // Retrieve $hash which is the last 32 characters of $decrypted.
    $hash = substr($decrypted, - 32);

    // Remove the last 32 characters from $decrypted.
    $decrypted = substr($decrypted, 0, - 32);

    // Integrity check. If this fails, either the data is corrupted, or the password/salt was incorrect.
    if (md5($decrypted) != $hash)
        return false;
        // Yay!
    return $decrypted;
}

// :: USAGE ::
$valueToEncrypt = 'hide_me_please!';
$secretToUse = 'mypassword';

echo "Value to encrypt: $valueToEncrypt\n";
echo "Secret to use: $secretToUse\n";

$hash = encrypt($valueToEncrypt, $secretToUse);
echo "Hashed value is: $hash\n\n";

$decryptedValue = decrypt($hash, $secretToUse);

if ($decryptedValue === $valueToEncrypt) {
    echo "Passed: $decryptedValue === $valueToEncrypt";
} else {
    echo "Failed: $decryptedValue !== $valueToEncrypt";
}

?>