Random ID code in php, of contact form [on hold]

I have php contact form in my html website, with fields like name, phone message, some dropdown options fields, etc... and it works well.

My question is, how to make a self generated code inside extra field in form. For example, when some one fills the form, he will get generated number code like 8893, which I will use later for some other reasons, like some ID of filled form.