PHP: Regex to get name="" 's value

I'm still very much learning Regex and I suck at it :(

I am getting an external webpage, saving it to a local file. The page contains a form, now I need to find all the form's input's names and put it into an array, for example:

<input type="text" name="firstName">

or

<input type="text" name="firstName" class="something">

from the above I just need firstName to go into my array.

To complicate matters just a little bit, I have noticed people writing it a bit different sometimes because of the space character, like so:

name= "firstName"
name = "firstName"
name ="firstName"

As I am still learning, I would really appreciate it if you could explain your regex