A dog that purrs and seems to have phlegm may be caused by an upper respiratory tract infection. If the situation is serious, you need to take the dog to the hospital for examination to avoid aggravation of the condition. If it is confirmed that it is an upper respiratory tract infection, you can give the dog some anti-inflammatory drugs or use atomization to treat it. In terms of diet, you need to strengthen nutrition and try to feed some light food. Generally, it will improve in about a week. If it does not improve, you need to be sent to the hospital for treatment.