AES-256 should not be confused with Rijndael-256.

PHP Mcrypt AES-256

<?php

	class Foo
	{

		const COOKIE_KEY = '839914B48AE583605F004C784FEF155B39A2C142AD7A747582160E6C14E7EF16';
		const COOKIE_IV = '864C9BB8032792391B88C76CF73276F2';

		public function encrypt( $what )
		{
			$cipher = MCRYPT_RIJNDAEL_128;
			$mode   = MCRYPT_MODE_CBC;
			$key    = hex2bin( self::COOKIE_KEY );
			$iv     = hex2bin( self::COOKIE_IV );

			return mcrypt_encrypt( $cipher, $key, $what, $mode, $iv );
		}

		public function decrypt( $what )
		{
			$cipher = MCRYPT_RIJNDAEL_128;
			$mode   = MCRYPT_MODE_CBC;
			$key    = hex2bin( self::COOKIE_KEY );
			$iv     = hex2bin( self::COOKIE_IV );

			return mcrypt_decrypt( $cipher, $key, $what, $mode, $iv );
		}
	}

	$f = new Foo();

	$str = bin2hex( $f->encrypt( 'hello there' ) );

	echo $f->decrypt( hex2bin( $str ) );