I don't know either, so let's find out together, by looking at the source-code for var_dump! (look for PHP_FUNCTION(var_dump)).

(For TL;DR, jump to the end)

1. The PHP function var_dump is a wrapper for the C function php_var_dump.

2. php_var_dump has a switch() statement to generate different output for each of PHP's basic types (numbers, strings, booleans, objects, etc), and we're interested in the object type.

3. Within the case IS_OBJECT: case we see this:

   php_printf("%sobject(%s)#%d (%d) {\n", COMMON, ZSTR_VAL(class_name), Z_OBJ_HANDLE_P(struc), myht ? zend_array_count(myht) : 0);
   

4. The #10 in your output comes from the #%d part of the format-string, which is the 3rd C variadic arg, and the (0) is the 4th C variadic arg.

   • the 3rd is Z_OBJ_HANDLE_P(struc)
   • the 4th is myht ? zend_array_count(myht) : 0

5. Z_OBJ_HANDLE_P basically returns a unique identifier for an object in PHP (so your PDO instance is the 11th object _{(I think, see below)} created in the processing of this request).

6. The myht thing is more complicated: but if it's set it means you asked PHP to var_dump an object member property (rather than an object itself), e.g. var_dump( $foo->bar ) instead of var_dump( $foo ). If you aren't referring to an object-property then it just prints 0.

Regarding how ->handle values are determined and what they mean:

• The Z_OBJ_HANDLE_P macro is Z_OBJ_HANDLE(*(zval_p))
  • The Z_OBJ_HANDLE macro is (Z_OBJ((zval)))->handle
  • The Z_OBJ macro is (zval).value.obj
  • So Z_OBJ_HANDLE_P(x) is the same as x.value.obj->handle
• Note that instances of user-defined classes and PHP "resources" are both "objects" and both have a uint32 handle member (but implemented separately).
  • You can see this if you compare _zend_object with _zend_resource in zend_types.h
• For "resources" (built-in objects provided by the PHP environment), PHP maintains a list of resources in an execution environment. When a new resource is created it's added to the list by zend_list_insert (which then uses the ZVAL_NEW_RES macro). The ->handle value is the index into that list (though I'm unsure if it starts at 0 or 1 or some other base).
• For PHP class objects ("user types", etc), the zend_objects_store_put function is used which adds the object to the objects_store list and returns the the index of the item in the list (so it's conceptually similar to zend_list_insert).
  • Again, I don't know what the initial or base value is (e.g. 0, 1, or something else).

TL;DR:

So object(PDO)#11 (0) { } means:

• the object is an instance of the PDO class.
• the object is the 11th _(probably) object created during the processing of the current HTTP request.
• the object is its own top-level object rather than an object property reference.