Php Tags:-

There are four different pairs of opening and closing tags which can be used in php. Two of those, <?php ?> and <script language="php"> </script>, are always available. The other two are short tags and ASP style tags, and can be turned on and off from the php.ini configuration file. As such, while some people find short tags and ASP style tags convenient, they are less portable, and generally not recommended. 

Note: Using short tags should be avoided when developing applications or libraries that are meant for redistribution, or deployment on PHP servers which are not under your control, because short tags may not be supported on the target server. For portable, redistributable code, be sure not to use short tags.

Reseved Variables:-
1.$_SERVER['HTTP_USER_AGENT']:- to check what sort of browser the visitor is using
2.$_POST['name']:- To get The  

Built In Function:- 
   1.strpos():- is a function built into PHP which searches a string for another string 
   2.var_dump():-If you want to check out the type and value of a certain expression, use it. 
   3.gettype(variable name):- prints out the type of varible 
       is_int()
       is_string()
   4.settype():- to set the type of the varible.. like Type cast.