I suggest the following notation:

```
Invalid value supplied: (integer) 12; Expected: date_mysql:/^\d{4}\-\d{2}\-\d{2}$/
```

This can be achieved through:

```php
<?php
$this->lastMessage = 'Invalid value supplied: (' . gettype($this->lastValue) . ') '
    . var_export($this->lastValue, true) . "; Expected: {$this}";
```