Will data order in post form be the same to it in web form?

Well, the W3C recommendation on HTML forms does say:

The control names/values are listed in the order they appear in the document.

Still, I'd consider it a bit risky to have your app depend critically on that detail.

PHP already handles converting POSTed/GETed variables into arrays when you put [] after the name. Do that instead of getting it wrong yourself.