Using checkbox in php problem

I add a checkbox in my page and will use the check result on another page. If it is checked, it has no problem. However, if it is not checked, it will have the checkbox unknown error. I read from an article that I can add a hidden field with the same name of the checkbox and put a value on this hidden field. It is OK on php. But it is not OK with the JavaScript. In the JavaScript, I check this: if(document.name.checkbox1.checked == true){ document.name.checkbox1.checked == false; } document.name.checkbox1.checked == false; But it cannot find out whether the box is checked or not. And the last line cannot uncheck the box neither. How to change the code will make it OK in both php and JavaScript? Thanks!!