rehash only in case something is changed.

#lowers the execution time in half on my machine on array with 1000 elements
#with nonscalar keys. Maybe it can be optimized further.

ext/standard/array.c

@@ -1866,7 +1866,9 @@ static void _phpi_pop(INTERNAL_FUNCTION_PARAMETERS, int off_the_end)
 			p = p->pListNext;
 		}
 		Z_ARRVAL_PP(stack)->nNextFreeElement = k;
-		zend_hash_rehash(Z_ARRVAL_PP(stack));
+		if (k) {
+			zend_hash_rehash(Z_ARRVAL_PP(stack));
+		}
 	} else if (!key_len) {
 		Z_ARRVAL_PP(stack)->nNextFreeElement = Z_ARRVAL_PP(stack)->nNextFreeElement - 1;
 	}