Enable backwards reading in list command

phpdbg_list.c

@@ -42,8 +42,9 @@ PHPDBG_LIST(lines) /* {{{ */
 		case NUMERIC_PARAM:
 		case EMPTY_PARAM:
 			phpdbg_list_file(phpdbg_current_file(TSRMLS_C),
-				param->type == EMPTY_PARAM ? 0 : param->num,
-				zend_get_executed_lineno(TSRMLS_C), 0 TSRMLS_CC);
+				param->type == EMPTY_PARAM ? 0 : (param->num < 0 ? 1 - param->num : param->num),
+				(param->type != EMPTY_PARAM && param->num < 0 ? param->num : 0) + zend_get_executed_lineno(TSRMLS_C),
+				0 TSRMLS_CC);
 			break;
 		case FILE_PARAM:
 			phpdbg_list_file(param->file.name, param->file.line, 0, 0 TSRMLS_CC);

phpdbg_prompt.c

@@ -668,6 +668,10 @@ PHPDBG_COMMAND(back) /* {{{ */
 			zval **file, **line, **funcname, **class, **type, **args;
 			char is_class;
 
+			if (limit < 0) {
+				phpdbg_error("Invalid backtrace size %d", limit);
+			}
+
 			zend_fetch_debug_backtrace(
 				&zbacktrace, 0, 0, limit TSRMLS_CC);
 

phpdbg_utils.c

@@ -69,7 +69,7 @@ PHPDBG_API int phpdbg_is_numeric(const char *str) /* {{{ */
         return 0;
 
 	for (; *str; str++) {
-		if (isspace(*str)) {
+		if (isspace(*str) || *str == '-') {
 			continue;
 		}
 		return isdigit(*str);