mcrypt_encrypt

<?php

# --- ENCRYPTION ---

# the key should be random binary, use scrypt, bcrypt or PBKDF2 to
# convert a string into a key
# key is specified using hexadecimal
$key = pack('H*', "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3");
# ??? This seems like 128 bits or 16 bytes to me (64 hex characters at 4 bits per character)
# ??? Why does the below print out 32 for the key size?
# ??? I thought you said below the key size should be 16 bit for AES-128.

# show key size use either 16, 24 or 32 byte keys for AES-128, 192
# and 256 respectively
$key_size =  strlen($key);
echo "Key size: " . $key_size . "\n";

$plaintext = "This string was AES-256 / CBC / ZeroBytePadding encrypted.";
# ??? Here you state that the string is encrypted with AES-256, but you
# ??? use MCRYPT_RIJNDAEL_128 below. Why?
# ??? Also what does ZeroBytePadding mean? As a newbie, I have no idea.

# create a random IV to use with CBC encoding
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

# creates a cipher text compatible with AES (Rijndael block size = 128)
# to keep the text confidential
# only suitable for encoded input that never ends with value 00h
# (because of default zero padding)
$ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key,
                             $plaintext, MCRYPT_MODE_CBC, $iv);
# ??? You say this is only suitable for encoded input that never ends with
# ??? value 00h. Is this something that should be checked? If so, can you
# ??? please provide code to catch this case? I'm not sure what 00h even means.

# prepend the IV for it to be available for decryption
$ciphertext = $iv . $ciphertext;

# encode the resulting cipher text so it can be represented by a string
$ciphertext_base64 = base64_encode($ciphertext);

echo  $ciphertext_base64 . "\n";

# === WARNING ===

# Resulting cipher text has no integrity or authenticity added
# and is not protected against padding oracle attacks.

# --- DECRYPTION ---

$ciphertext_dec = base64_decode($ciphertext_base64);

# retrieves the IV, iv_size should be created using mcrypt_get_iv_size()
$iv_dec = substr($ciphertext_dec, 0, $iv_size);

# retrieves the cipher text (everything except the $iv_size in the front)
$ciphertext_dec = substr($ciphertext_dec, $iv_size);

# may remove 00h valued characters from end of plain text
$plaintext_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key,
                                $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);

echo  $plaintext_dec . "\n";

?>