html - Keep value data in form after submiting PHP

I have had a look at other posts but could not solve the issue.

I am trying to find what is the easiest way to keep form values after post. For example a registration form,

If a user enters all correct data except for one field (an email address for example) then the form gets submitted via a PHP form, how do I keep the valid form data but get rid of the data that is not valid?

Below is my form code:

        Your Name: 
             

        Username:   
            

        Email:
             

        Password:

            


        Age:
            

            Are you Male or Female?
                 Male
                 Female



            What gender are your dogs?
                 Male     
                 Female