How can we generate auto increment invoice #

Question

   $runningnumber=1;
   $comp = "CT";
   echo $result = $comp.date('mdy').str_pad($runningnumber++, 5, "0", 
   STR_PAD_LEFT);

how do i increase the $runnungnumber like 00001, 00002, 00003, 00004, 00005, 00006 .... please help

my desired output is

CT08221700001
CT08221700002
CT08221700003
CT08221700004
CT08221700005