Here is a simple solution without any loops or any hassle which will allow you to create random string with characters, numbers or even with special symbols.
$randomNum = substr(str_shuffle("0123456789"), 0, $x);
where $x
can be number of digits
Eg.
substr(str_shuffle("0123456789"), 0, 5);
Results after a couple of executions
98450
79324
23017
04317
26479
You can use the same code to generate random string also, like this
$randomNum=substr(str_shuffle("0123456789abcdefghijklmnopqrstvwxyzABCDEFGHIJKLMNOPQRSTVWXYZ"), 0, $x);
Results with $x = 11
FgHmqpTR3Ox
O9BsNgcPJDb
1v8Aw5b6H7f
haH40dmAxZf
0EpvHL5lTKr