Well, a loop is a loop, but there is a huge difference between PHP and .Net. Fortunately for you PHP is easy to learn and use so if you understand the fundamentals of programming you're learning curve shouldn't be too steep. Especially if you know C# which is Java-like as PHP "borrows" a lot from Java.

I think you should consult the php site. Some useful resources:
http://www.php.net/manual/en/tutorial.php
http://www.php.net/manual/en/langref.php
http://www.php.net/manual/en/security.php
http://www.php.net/manual/en/features.php
http://www.php.net/manual/en/funcref.php

i moved to php from asp.net
and believe me it was not so hard
but php opens your mind so much
in the other hand Microsoft tries to make the programming easy but on its way , it is gonna be hard if u want to work .net ur way
anyway welcome to php world