Would mydomain.com/key_phrase_here.php be better for search engines than mydomain.com/keyphrasehere.php? I saw a match for the second method, someone following in a link from google after searching for a keyphrase where they hadn't entered any spaces. Not the usual way to search, but it got me thinking whether underscores would make any difference?
Are underscores treated as spaces by SEs? For example, if I wanted to promote the keyword "e-mail", knowing more users type that than "email", would I still be better using the keyword "email" on my pages, the idea being "e-mail" will be picked up as two separate words in SEs, rather than being seen as joined?
Underscores are not treated as spaces by SEs. They are treated as underscores
The Golden Rule here is that most of the time, SEs will reward you for taking actions that make real live visitors' lifes better. It's easier to read a page name with underscores, and yes, it just so happens most SEs slightly favor this approach. Maybe it's coincidence, but it's a good general rule to follow.
As for the e-mail v. email bit, I have no idea. Good question though!