Get URL parameter name in PHP -

i working on php controller, url:

www.example.com?field1=test1&field2=test2&field3=test3

i know can value this:

$_get['field1'] // return test1

but need return name of field, in case field1, i'm wondering if can loop through $_get variable, i'm not sure how.

$_post array. can loop through this:

foreach($_post $key=>$value) {   echo "$key=$value"; }

there's array_keys function might of use.

And