I checked similar questions, but I am still confused as a beginner and would like a simple explanation.

I am a beginner learning PHP and working with HTML forms.

I am submitting a form using the POST method, but when I try to access the form values in PHP, I get a warning like:

Undefined index: username

I understand this happens when the index is not found, but I am not sure what is the correct and safe way to handle this as a beginner.

What is the proper way to check and access form values in PHP to avoid this warning?

HTML:

    
    Submit


PHP: