如何检查html表单中的必填字段是否已填写？

I've got a submission page in php with an html form that points back to the same page. I'd like to be able to check if the required fields in the form aren't filled so I can inform the user. I'd like to know how to do that with php and javascript each. However, I imagine this is a common issue so any other answers are welcome.I've got a submission page in php with an html