AES encryption and decryption class sharing written by php

Today, I wrote an AES encryption class for php. Applicable to extensions of Yii.
If not in the Yii framework, put Yii:: app ()- > Simply replace params ['encryptKey'] with your default key.
Class code:

<?php
/**
 * php AES Encryption and decryption class 
 *  If you want to work with java Share, the key length should be 16 Bit length 
 *  Because java Support only 128 Bit encryption, so php Also use 128 Bit encryption, which can be used with java Turn each other. 
 *  Simultaneously AES The standard is also 128 Bit. It's just RIJNDAEL The algorithm can support 128 , 192 And 256 Bit encryption. 
 * java  To use the AES/CBC/NoPadding Standard to encrypt and decrypt 
 * 
 * @author Terry
 *
 */
class PhpAes
{
	/**
	 * This was AES-128 / CBC / NoPadding encrypted.
	 * return base64_encode string
	 * @author Terry
	 * @param string $plaintext
	 * @param string $key
	 */
	public static function AesEncrypt($plaintext,$key = null)
	{
		$plaintext = trim($plaintext);
		if ($plaintext == '') return '';
		if(!extension_loaded('mcrypt'))
			throw new CException(Yii::t('yii','AesEncrypt requires PHP mcrypt extension to be loaded in order to use data encryption feature.'));
		$size = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
		$module = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');
		$key=self::substr($key===null ? Yii::app()->params['encryptKey'] : $key, 0, mcrypt_enc_get_key_size($module));
		/* Create the IV and determine the keysize length, use MCRYPT_RAND
		 * on Windows instead */
		$iv = substr(md5($key),0,mcrypt_enc_get_iv_size($module));
		/* Intialize encryption */
		mcrypt_generic_init($module, $key, $iv);

		/* Encrypt data */
		$encrypted = mcrypt_generic($module, $plaintext);

		/* Terminate encryption handler */
		mcrypt_generic_deinit($module);
		mcrypt_module_close($module);
		return base64_encode($encrypted);
	}

	/**
	 * This was AES-128 / CBC / NoPadding decrypted.
	 * @author Terry
	 * @param string $encrypted		base64_encode encrypted string
	 * @param string $key
	 * @throws CException
	 * @return string
	 */
	public static function AesDecrypt($encrypted, $key = null)
	{
		if ($encrypted == '') return '';
		if(!extension_loaded('mcrypt'))
			throw new CException(Yii::t('yii','AesDecrypt requires PHP mcrypt extension to be loaded in order to use data encryption feature.'));

		$ciphertext_dec = base64_decode($encrypted);
		$module = mcrypt_module_open(MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');
		$key=self::substr($key===null ? Yii::app()->params['encryptKey'] : $key, 0, mcrypt_enc_get_key_size($module));

		$iv = substr(md5($key),0,mcrypt_enc_get_iv_size($module));

		/* Initialize encryption module for decryption */
		mcrypt_generic_init($module, $key, $iv);

		/* Decrypt encrypted string */
		$decrypted = mdecrypt_generic($module, $ciphertext_dec);

		/* Terminate decryption handle and close module */
		mcrypt_generic_deinit($module);
		mcrypt_module_close($module);
		return rtrim($decrypted,"\0");
	}

	/**
	 * Returns the length of the given string.
	 * If available uses the multibyte string function mb_strlen.
	 * @param string $string the string being measured for length
	 * @return integer the length of the string
	 */
	private static function strlen($string)
	{
		return extension_loaded('mbstring') ? mb_strlen($string,'8bit') : strlen($string);
	}

	/**
	 * Returns the portion of string specified by the start and length parameters.
	 * If available uses the multibyte string function mb_substr
	 * @param string $string the input string. Must be one character or longer.
	 * @param integer $start the starting position
	 * @param integer $length the desired portion length
	 * @return string the extracted part of string, or FALSE on failure or an empty string.
	 */
	private static function substr($string,$start,$length)
	{
		return extension_loaded('mbstring') ? mb_substr($string,$start,$length,'8bit') : substr($string,$start,$length);
	}
}