Random string in PHP not enough characters

Question

I'm trying to make some random string in PHP with 5 letters/numbers. It's working fine but sometimes I get a shorter string.

$characterset='ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
$count=0;
$lenght=strlen($characterset);
$i=5; #number of characters
while($count < $i){
  $num=rand(1,$lenght);
  $letter=substr($characterset, $num, 1);
  $string.=$letter;
  $count++;
}

And strlen($string) is sometimes 4 (checked 100 records, 85 was 4 characters)

anyway, why it is only sometimes not correct? 99% records were good — hyperster666, Jul 05 '17 at 13:13
Because `substr` silently returns false, if you try to get a character from a position beyond the string end ... — CBroe, Jul 05 '17 at 13:15
Try [this](https://stackoverflow.com/questions/48124/generating-pseudorandom-alpha-numeric-strings). — Jaydeep Mor, Jul 05 '17 at 13:16
https://stackoverflow.com/questions/5438760/generate-random-5-characters-string — Nadeem Shaikh, Jul 05 '17 at 13:16
You might like to fix the `Notice: Undefined variable: string` by declaring `$string = '';'` just outside the loop. `.=` operators only work properly on pre initialised variables — RiggsFolly, Jul 05 '17 at 13:18

score 3 · Answer 1 · answered Jul 05 '17 at 13:15

String characters, like arrays, start counting from zero. Run your code a bunch of times: in addition to sometimes getting not enough characters, notice how you never get an A in there?

$num = rand(0,$lenght-1); will do it.

As an alternative method, you could do this:

$max_b36 = str_repeat("Z",$number_of_characters);
$max_dec = base_convert($max_b36,36,10);
$rand_dec = rand(0,$max_dec);
$rand_b36 = base_convert($rand_dex,10,36);
$result = sprintf("%0".$number_of_characters."s",$rand_b36);

This method uses (potentially) big numbers though, so it will only support 5 characters (on 32-bit systems) or 12 characters (on 64-bit systems).

score 1 · Answer 2 · answered Jul 05 '17 at 13:24

1

Personally I'd replace your entire logic with this one line wonder :

print $characters = substr(
                          str_shuffle('ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890'),
                          0,5)
                          );

(slightly off formatting so it fits on the screeen)

answered Jul 05 '17 at 13:24

Doug

1,850
23
50

Worth noting that this will generate strings with no duplicate characters. Pretty cool, though! – Niet the Dark Absol Jul 05 '17 at 13:29

score 0 · Answer 3 · answered Jul 05 '17 at 14:09

Thank you all for help, following code working great:

$characters='ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
$string = '';
$random_string_length = 5;
$max = strlen($characters) - 1;
for ($i = 0; $i < $random_string_length; $i++) {
     $string .= $characters[mt_rand(0, $max)];
}

Is it possible to avoid the same strings? I generated 50 000 records and had 48 the same.

Random string in PHP not enough characters

3 Answers3