Dreaming of ciphers

function my_stream_cipher($plaintext, $key)
{
  $ciphertext = $plaintext;
  $sha = sha1($key);
 
  for ($i = 0; $i < 5; $i++)
  {
    mt_srand( hexdec( substr($sha, ( $i * 8 ), 8 ) ) );
    for ($j = 0; $j < strlen($plaintext); $j++)
      $ciphertext{$j} = chr( ord( $ciphertext{$j} ) ^ mt_rand(0, 255) );
  }
 
  return $ciphertext; 
}

I was trying to sleep this morning but was harassed endlessly by a really simple cipher that wouldn't stop running simulations in my head. It seemed like it'd be partially useful for some PHP projects I'm working on so I got up, fired up Aptana, and typed it up. A little google-fu revealed it to be a 5-pass vernam cipher.

This is how it works, in a nutshell:

1. Set current ciphertext to plaintext
2. Break the SHA1 of the key into 5 individual 32-bit ints
3. For each int:
   1. Seed the RNG with the int to generate a stream of random characters
   2. XOR the current ciphertext with the stream

Can anyone spot the obvious flaw in this approach?