Zend Framework generate unique string

I want to generate a unique 4-6 char long AlphaNumeric string to save in db with each record(user). The db field has a unique index, so trying to save a pre-existing string generates an error. Right now I am generating a random string and using try-catch, so when adding a new record if it throws an exception, I generate another random string and attempt to save again, and the code keep trying until it adds a record successfully. This whole solution not only looks heavy but also ugly, so I want to change it. I am interested in an elegant solution, so any help/guidance is welcome.

Solution

With the given information :

id must be unique
id must not be numeric
id must not represent a sequential series
id will not be input by the user

The PHP function uniqid is exactly what you need. Though it returns a 13 character long hexadecimal value.

** Edit **

Yes, uniqid will return a seamingly sequential number, but we can get around this easily. Consider this code

class IDGenerator {
   //const BIT_MASK = '01110011';

   static public function generate() {

      $id = uniqid();

      $id = base_convert($id, 16, 2);
      $id = str_pad($id, strlen($id) + (8 - (strlen($id) % 8)), '0', STR_PAD_LEFT);

      $chunks = str_split($id, 8);
      //$mask = (int) base_convert(IDGenerator::BIT_MASK, 2, 10);

      $id = array();
      foreach ($chunks as $key => $chunk) {
         //$chunk = str_pad(base_convert(base_convert($chunk, 2, 10) ^ $mask, 10, 2), 8, '0', STR_PAD_LEFT);
         if ($key & 1) {  // odd
            array_unshift($id, $chunk);
         } else {         // even
            array_push($id, $chunk);
         }
      }

      return base_convert(implode($id), 2, 36);
   }
}

echo IDGenerator::generate();

Which will give results like

ivpa493xrx7
d173barerui
evpoiyjdryd
99ej19mnau2

Since there is nothing added or modified, except shuffling the bits around, there should not be any duplicated values and everything seems random. Voilà!

Update (2014-02-24)

I update this piece of code since the time it was originally posted. You may find the revised version here

Zend Framework generate unique string

** Update (2014-02-24) **

Update (2014-02-24)