escape_html generates incorrect php code

[wxiaoguang]

When setting escape_html=true, the following code:

{$foo+$bar}

becomes:

<?php echo htmlspecialchars((string) $_smarty_tpl->tpl_vars['foo']->value+$_smarty_tpl->tpl_vars['bar']->value, ENT_QUOTES, 'UTF-8');?>

The code: (string) $a + $b is not correct, maybe it should be (string) ($a + $b).