First attempt at entropy estimator

<?
/* So I wrote a password entropy guessing algorithm today.
 * It looks for common patterns and weaknesses in the keyspace of the password (repeat characters, repeats
 * of the same character type, etc.) by analyzing different chunks of the password.
 *
 * This is probably not the most efficient way to do this...
 */
function getEntropy($p) {
  # Estimates the average entropy per segment of the password :)
  $l = strlen($p);
  $estimates = array();
  for($i = 0; $i <= $l/2; $i++) {
    $chnk = str_split($p, $i+1);
    $estimates[$i] = 0;
    $prevchunk = '';
    foreach($chnk as $c) {
      # Look for really weak patterns in different segments of the string!
      if($prevchunk == $c) continue;
      if(preg_match("/(.)\\1{".($i-1)."}/", $c, $m)) {
        # No entropy gain for repeated characters.
        if(preg_match('/^[0-9]+$/', $m[1])) {
          $estimates[$i] += 3.3219281;
        } elseif(preg_match('/^[0-9]$/', $m[1])) {
          $estimates[$i] += 3.3219281;
        } elseif(preg_match('/^[a-z]$/', $m[1]) || preg_match('/^[A-Z]$/', $m[1])) {
          $estimates[$i] += 4.7004397;
        } elseif(preg_match('/^[^a-zA-Z0-9]$/', $m[1])) {
          $estimates[$i] += 5.4262648;
        } else {
          # Best case
          $estimates[$i] += 6.5698556;
        }
      } elseif(preg_match('/^[0-9]+$/', $c)) {
        $estimates[$i] += ($i+1)*3.3219281;
      } elseif(preg_match('/^[a-z]+$/', $c) || preg_match('/^[A-Z]+$/', $c)) {
        $estimates[$i] += ($i+1)*4.7004397;
      } elseif(preg_match('/^[A-Za-z]+$/', $c)) {
        $estimates[$i] += ($i+1)*5.7004397;
      } elseif(preg_match('/^[^a-zA-Z0-9]+$/', $c)) {
        $estimates[$i] += ($i+1)*5.4262648;
      } elseif(preg_match('/^([A-Z]+)([a-z]+)([0-9]*)$/', $c, $m)) {
        $estimates[$i] += strlen($m[1])*4.7004397;
        $estimates[$i] += strlen($m[2])*4.7004397;
        $estimates[$i] += strlen($m[3])*3.3219281;
      } else {
        # Best case
        $estimates[$i] += ($i+1)*6.5698556;
      }
      $prevchunk = $c;
    }
    if($estimates[$i] == 0) unset($estimates[$i]);
  }
  if(empty($estimates)) return 0;
  return min($estimates); // Yes!
}
?>
<h1>Entropy Estimate</h1>
<form>Enter Password:<input type="text" style="width: 1000px;" name="password" value="<?=empty($_GET['password'])?'':stripXSS($_GET['password']); ?>" /> <input type="submit" value="Go" /></form>
<hr />
<?
if(!empty($_GET['password'])) {
  $p = $_GET['password'];
  $l = strlen($p);
  if($l < 4) {
    echo "Password too short!";
    exit;
  }
  $E = getEntropy($p);
  echo "Maximum entropy: ".number_format($E, 2)." bits of entropy.<br />\n";
  $N = bcpow('2', $E);
  echo "There are approximately ".number_format($N)." passwords in this keyspace.<br />\n";
  echo "At 900 Quadrillion passwords per second, it would take approximately ";
  $T = bcdiv($N, '900000000000000000');
  if($T < 60) {
    echo $T." seconds";
  } elseif($T < 3600) {
    echo bcdiv($T, 60)." minutes, ".bcmod($T, 60)." seconds";
  } elseif($T < 86400) {
    echo bcdiv($T, 3600)." hours, ".floor(bcdiv(bcmod($T, 3600), 60))." minutes, ".bcmod($T, 60)." seconds";
  } elseif($T < 31556736) {
    echo bcdiv(bcmod($T, 86400), 3600)." hours, ".bcdiv(bcmod($T, 3600), 60)." minutes, ".floor(bcmod($T, 60))." seconds";
  } else {
    echo number_format(bcdiv($T, 31556736))." years, ".floor(bcdiv(bcmod($T, 31556736), 86400))." days, ".floor(bcdiv(bcmod($T, 86400), 3600)." hours, ".bcdiv(bcmod($T, 3600), 60))." minutes, ".floor(bcmod($T, 60))." seconds";
  }
  echo " to guess your password.";
}
?>