PHP generates random numbers with custom function randstr($length)

function randstr($len=6){ 
    $chars='abcdefghijklmnopqrstuvwxyz0123456789'; 
    #characters to build the password from 
    mt_srand((double)microtime()*1000000*getmypid()); 
    #seed the random number generater (must be done) 
    $password=''; 
    while(strlen($password)<$len) 
        $password.=substr($chars,(mt_rand()%strlen($chars)),1); 
    return $password; 
} 

$salt = randstr();