Sorry, but your "URL" conversion is not homogeneous (= impossible to "generalize" because they do not share the same principles exactly ), so best of all I could do:
RewriteEngine On ^/?$ index.php [NC,QSA,L] ^/?about$ about.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)$ /$1.php [NC,QSA,L] ^/?(publisher|agencies|advertiser)/sign_up$ /$1_signup.php [NC,QSA,L] ^/?(publisher|agencies|advertiser)/login$ /login.php [NC,QSA,L]
But if you want “homogeneous” things that don't match your needs, you can have “cleaner” rules, for example:
RewriteEngine On ^/?$ index.php [NC,QSA,L] ^/?about$ about.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)$ /$1.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)/sign_up$ /$1_signup.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)/login$ /login.php [NC,QSA,L]
And if you really want to centralize everything, you can do (= pass the type to the php file):
RewriteEngine On ^/?$ index.php [NC,QSA,L] ^/?about$ about.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)$ /$1.php [NC,QSA,L] ^/?(agencies|contact|publisher|advertiser)/(sign_up|login)$ /$2.php?type=$1 [NC,QSA,L]