Detailed explanation of PHP's AES encryption algorithm example

<?php
class MCrypt {
  private $hex_iv = &#39;00000000000000000000000000000000&#39;; # converted JAVA byte code in to HEX and placed it here
  private $key = &#39;U1MjU1M0FDOUZ.Qz&#39;; #Same as in JAVA
  function __construct() {
    $this->key = hash(&#39;sha256&#39;, $this->key, true);
    //echo $this->key.&#39;<br/>&#39;;
  }
  function encrypt($str) {
    $td = mcrypt_module_open(MCRYPT_RIJNDAEL_128, &#39;&#39;, MCRYPT_MODE_CBC, &#39;&#39;);
    mcrypt_generic_init($td, $this->key, $this->hexToStr($this->hex_iv));
    $block = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $pad = $block - (strlen($str) % $block);
    $str .= str_repeat(chr($pad), $pad);
    $encrypted = mcrypt_generic($td, $str);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return base64_encode($encrypted);
  }
  function decrypt($code) {
    $td = mcrypt_module_open(MCRYPT_RIJNDAEL_128, &#39;&#39;, MCRYPT_MODE_CBC, &#39;&#39;);
    mcrypt_generic_init($td, $this->key, $this->hexToStr($this->hex_iv));
    $str = mdecrypt_generic($td, base64_decode($code));
    $block = mcrypt_get_block_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    mcrypt_generic_deinit($td);
    mcrypt_module_close($td);
    return $this->strippadding($str);
  }
  /*
   For PKCS7 padding
   */
  private function addpadding($string, $blocksize = 16) {
    $len = strlen($string);
    $pad = $blocksize - ($len % $blocksize);
    $string .= str_repeat(chr($pad), $pad);
    return $string;
  }
  private function strippadding($string) {
    $slast = ord(substr($string, -1));
    $slastc = chr($slast);
    $pcheck = substr($string, -$slast);
    if (preg_match("/$slastc{" . $slast . "}/", $string)) {
      $string = substr($string, 0, strlen($string) - $slast);
      return $string;
    } else {
      return false;
    }
  }
  function hexToStr($hex)
  {
    $string=&#39;&#39;;
    for ($i=0; $i < strlen($hex)-1; $i+=2)
    {
      $string .= chr(hexdec($hex[$i].$hex[$i+1]));
    }
    return $string;
  }
}
$encryption = new MCrypt();
echo $encryption->encrypt(&#39;123456&#39;) . "<br/>";
echo $encryption->decrypt(&#39;tpyxISJ83dqEs3uw8bN/+w==&#39;);
?>