Intention of Salaah
It is not necessary to make a verbal intention. If a person thinks in his heart that I am reading the fard of zuhr for today, or if it is a sunnah, then I am reading the sunnah of zuhr; then this will be sufficient. All those lengthy intentions which are popular among the people are not necessary at all.
If one wants to make a verbal intention, it is sufficient to say thus: I am making intention for todays fard of zuhr, or, I am making intention for the sunnah of zuhr. To say: I am reading four rakaats for zuhr salaat, facing the qiblah, etc. is not necessary. If one wishes, he could say so, if not, then he does not have to.
In his heart a person has the intention of zuhr salaat, but when he utters his intention, he mistakenly says asr salaat. Even then his salaat will be valid.
If he mistakenly says six rakaats or three rakaats instead of four rakaats, his salaat will still be valid.
A person offered zuhr salaat. On completing his salaat, he realized that the zuhr time had already expired and asr time had already entered. He will not have to make qada of that salaat. The salaat which he had offered will come under the rule of qada and it will be considered to be a qada salaat. But if he offered a salaat even before the entry of its time, it will not be valid.