Using while() and continue in PHP - php

Question

I am learning PHP and trying to use the while and continue expressions correctly.
I have a script that creates a 6 digit PIN, and I want to ensure that it is unique, otherwise I want to generate another PIN.
while(1) {
$pin = rand(111111,999999);
$sel = mysql_query("SELECT * FROM formusers WHERE pin = '$pin'");
if(mysql_num_rows($sel) != 0) { continue; }
mysql_query("INSERT INTO formusers(email,password,pin) VALUES('".$_POST['srEmail']."','".$_POST['srPass']."','".$pin."')");
if(mysql_affected_rows()!=-1) {
echo "Pin:" . $pin;
exit;
} else {
echo "Existing email, try again<br />";
}
break;
}
Do I have the syntax for the loop correct? It seems to work, but there's no way for me to debug it in the instance the rand() function creates the same PIN twice.
Also, in the event I did run out of unique PINs, what would happen here? Presumably it would loop indefinitely? Is there any way to prevent this?

Answer 1

Yeah, the code works, but as stated the infinite loop is concerning. You could possibly solve that like this:
var $i = 0;
while($i < 10) {
$i++;
$pin = rand(111111,999999);
$sel = mysql_query("SELECT * FROM formusers WHERE pin = '$pin'");
if(mysql_num_rows($sel) != 0) { continue; }
mysql_query("INSERT INTO formusers(email,password,pin) VALUES('".$_POST['srEmail']."','".$_POST['srPass']."','".$pin."')");
if(mysql_affected_rows()!=-1) {
echo "Pin:" . $pin;
exit;
} else {
echo "Existing email, try again<br />";
}
break;
}
This would ensure you would never make more than 10 iterations. However, the larger problem is probably that even that sampling size won't work in the future. This presents a bit of a conundrum. One possible approach is to determine how many unique numbers exist before starting the loop by counting them and then iterate that many times, but that approach is something like n2 (maybe, I'm not real good on big-O, I just know it would be bad).

Answer 2

You can leverage the unique nature of php array keys for this.
function getPins($qty){
$pins = array();
while (count($pins) < $qty){
$pins[rand(111111,999999)] = "";
}
return array_keys($pins);
}
This will make sure that for a given run you will get $qty number of unique pins. You might want to look at appending a unique prefix to each run of the function to avoid multiple runs creating collisions between the two.

Answer 3

You can seed random, so it gives you the same values every time, you run it:
srand (55 ); //Or some other value.
The while loop syntax looks okay to me.