[PHP] fix __toString for nested models

[wing328]

To fix #1914

{
    "id": 10005,
    "category": {
        "id": 10005,
        "name": "test php category"
    },
    "name": "PHP Unit Test",
    "tags": [
        {
            "id": 10005,
            "name": "test php tag"
        }
    ]
}