blank form

in my php scripts I am using forms which are posted to itself, each time fields are submitted the php scripts checks the syntax before moving onto the next part of the form, if there is an error the script will tell the user that there is an error and to use the back button to correct the form, problem is when you press the back button the form is blank. how do I overcome this by keeping the original details in the form especially when using drop down menu options ??