The enter key submits your form, but as your form does not have an
input with name
product_id, the PHP code does not find it and generates the exception.
Now it depends on what you want:
Do you want the form submission system? Then make sure your PHP code can deal with that, and does not expect the
product_idto always be defined in the
$_GETarray, but instead some variables (for named
inputelements) in the
Do you not want the form to be submitted? Then either don't use the
formtag, or else add a
onsubmit="return false"attribute to it.