how can i combine number php

hello all my name is rico,
i want make some app like can combine number from input
example :
i have 4 variable like this
$num1 = 96;
$num2 = 20;
$num3 = 19;
$num4 = 37;

so how can i get the result like this
9213 9217 9293 9297 .. etc.

and example 2 if $num is empty then the result is like this
213 217 293 .. etc.

note : space in number is for star i can't write start in simbol in there