How to generate random unique string in php

Here I am using these function one function getting the information from the database so that I can confirm each generated  strings will be unique.

   function isToken($token)
    {
        if (isset($token) && $token) {
         // Your query herer
      }
}

function generateUniqueToken($number=10)
{
    $arr = array('A', 'B', 'C', 'D', 'E', 'F',
                 'G', 'H', 'I', 'J', 'K', 'L',
                 'M', 'N', 'O', 'P', 'R', 'S',
                 'T', 'U', 'V', 'X', 'Y', 'Z',
                 '1', '2', '3', '4', '5', '6',
                 '7', '8', '9', '0');
    $token = "";
    for ($i = 0; $i < $number; $i++) {
        $index = rand(0, count($arr) - 1);
        $token .= $arr[$index];
    }

    if (isToken($token)) {
        return generateUniqueToken($number);
    } else {
        return $token;
    }
}