[an0nym@localhost ~]$ cat test.php 
<?php
 switch  ($argv[ 1 ])
{
   case  'berminga':
     function  generateId($len)
    {
      $letters="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
      
      $str = "";
       for  ( $i =  0 ; $i < $len; $i = $i+ 1  )
        $str = $str.$letters[  rand ()% 62  ];

       return  $str;
    }
     break ;
   case  'an0nym':
     function  generateId($length)
    {
      $alphabet = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
      $alphabet_length =  62 ;
       return   substr ( str_shuffle ( str_repeat ($alphabet,  ceil ($length / $alphabet_length))),  0 , $length);
    }
     break ;
}

 const  LENGTH_MAX =  1024 ;
 const  TRIES =  1000000 ;

 for  ($length =  1 ; $length <= LENGTH_MAX; $length *=  2 ) {
  $start =  microtime ( true );
   for  ($i =  0 ; $i < TRIES; ++$i)
    generateId($length);
  $end =  microtime ( true );

   echo  $length, "\t", $end - $start, "\n";
}
[an0nym@localhost ~]$ php test.php berminga
 1 	 1 . 2689220905304 
 2 	 1 . 8100938796997 
 4 	 2 . 7736878395081 
 8 	 4 . 8485329151154 
 16 	 8 . 8386499881744 
 32 	 16 . 961936950684 
 64 	 33 . 190233945847 
^C
[an0nym@localhost ~]$ php test.php an0nym
 1 	 3 . 6046829223633 
 2 	 3 . 5677490234375 
 4 	 3 . 624372959137 
 8 	 3 . 6216700077057 
 16 	 3 . 5305140018463 
 32 	 3 . 6543760299683 
 64 	 4 . 8884370326996 
 128 	 6 . 240996837616 
 256 	 9 . 3746719360352 
 512 	 14 . 413372039795 
 1024 	 24 . 756990909576