A simple method to encrypt or decrypt a plain text string in PHP.

<?php

/**
 * PHP encrypt and decrypt example
 *
 * Simple method to encrypt or decrypt a plain text string initialization
 * vector(IV) has to be the same when encrypting and decrypting in PHP 5.4.9.
 *
 * @link http://naveensnayak.wordpress.com/2013/03/12/simple-php-encrypt-and-decrypt/
 *
 * @param string $action Acceptable values are `encrypt` or `decrypt`.
 * @param string $string The string value to encrypt or decrypt.
 * @return string
 */
function encrypt_decrypt($action, $string)
{
	$output = false;

	$encrypt_method = "AES-256-CBC";
	$secret_key = 'This is my secret key';
	$secret_iv = 'This is my secret iv';

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

	// iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a
	// warning
	$iv = substr(hash('sha256', $secret_iv), 0, 16);

	if ($action == 'encrypt')
	{
		$output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
		$output = base64_encode($output);
	}
	else
	{
		if ($action == 'decrypt')
		{
			$output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
		}
	}

	return $output;
}


//
// usage
//

$plain_txt = "This is my plain text";
echo "Plain Text = $plain_txt\n";

$encrypted_txt = encrypt_decrypt('encrypt', $plain_txt);
echo "Encrypted Text = $encrypted_txt\n";

$decrypted_txt = encrypt_decrypt('decrypt', $encrypted_txt);
echo "Decrypted Text = $decrypted_txt\n";

if ($plain_txt === $decrypted_txt)
	echo "SUCCESS";
else
	echo "FAILED";

echo "\n";

?>