Consolidate UNSET_DIM handling for string offset error

The immediate error here is the nested indexing in write context,
the fact that it's ultimately wrapped in an unset() doesn't matter.
Same as $str[0][0] += 1 will throw "Cannot use string offset as an
array", so should this case.

Author: nikic

Zend/tests/bug70089.phpt

@@ -30,6 +30,6 @@ try {
 ?>
 --EXPECT--
 string(36) "Cannot use string offset as an array"
-string(27) "Cannot unset string offsets"
+string(36) "Cannot use string offset as an array"
 string(47) "Cannot create references to/from string offsets"
 string(41) "Cannot increment/decrement string offsets"

Zend/tests/unset_non_array.phpt

@@ -109,5 +109,5 @@ Deprecated: Automatic conversion of false to array is deprecated in %s
 Cannot unset offset in a non-array variable
 Cannot unset offset in a non-array variable
 Cannot unset offset in a non-array variable
-Cannot unset string offsets
+Cannot use string offset as an array
 Cannot use object of type stdClass as array

Zend/zend_execute.c

@@ -1573,6 +1573,7 @@ ZEND_API ZEND_COLD void zend_wrong_string_offset_error(void)
 					case ZEND_FETCH_LIST_W:
 					case ZEND_ASSIGN_DIM:
 					case ZEND_ASSIGN_DIM_OP:
+					case ZEND_UNSET_DIM:
 						msg = "Cannot use string offset as an array";
 						break;
 					case ZEND_PRE_INC:
@@ -1594,9 +1595,6 @@ ZEND_API ZEND_COLD void zend_wrong_string_offset_error(void)
 					case ZEND_FE_RESET_RW:
 						msg = "Cannot create references to/from string offsets";
 						break;
-					case ZEND_UNSET_DIM:
-						msg = "Cannot unset string offsets";
-						break;
 					EMPTY_SWITCH_DEFAULT_CASE();
 				}
 				break;