const int* ptr;
declares ptr a pointer to const int type. You can modify ptr itself but the object pointed to by ptr shall not be modified.
const int a = 10;
const int* ptr = &a;
*ptr = 5; // wrong
ptr++; // right
While
int * const ptr;
declares ptr a const pointer to int type. You are not allowed to modify ptr but the object pointed to by ptr can be modified.
int a = 10;
int *const ptr = &a;
*ptr = 5; // right
ptr++; // wrong
Generally I would prefer the declaration like this which make it easy to read and understand (read from right to left):
int const *ptr; // ptr is a pointer to constant int
int *const ptr; // ptr is a constant pointer to int