Global variables should be named in uppercase.

Variables with a large scope should have long names, variables with a small scope can have short names. Temporary variables are best kept short. Someone reading such variables should be able to assume that its value is not used outside a few lines of code.

Common temporary variables are $i, $j, $k, $m and $n. Since these variables should have small scope prefixes are a bit of overkill.

Always access GET and POST vars by a TYPO3 API function (and directly through HTTP_GET_VARS or $_GET or the same for POST)

If you need to set a value in HTTP_GET_VARS globally, always use t3lib_div::_GETset() to write back the HTTP_GET_VARS array (will set both HTTP_GET_VARS and the superglobal $_GET)

If you access values directly in HTTP_GET_VARS/HTTP_POST_VARS you MUST only do so if your code acts if there are no slashes on the values! (It is not allowed to check magic_quotes settings since TYPO3 will normalize the arrays no matter what!)

It is also acceptable to use HTTP_GET_VARS and HTTP_POST_VARS directly if you use such as isset() to check whether a value is set.

If possible, group requests after input variables in an init() function of your classes - thus it is clear to see what external input your application expects. (There are multiple examples in the core of the init() function loading all input variables into internal vars of the class by a series of t3lib_div::_GP() function calls!)

Do NOT expect in your scripts that input from outside is found in the global vars. It's recommended to use the php.ini-optimized with TYPO3 and that means that the content of HTTP_POST/GET_VARS is NOT transferred to the global name-space.

Generally keep variables safe inside of classes in whatever work you do with TYPO3.

Use a namespace for your GET and POST variables in extensions:If you want to make a form-field with the email address, name it:

<input name="tx_myext[email]"> or <input name="tx_myext[DATA][email]">

Apply the same principles of naming on GET vars as well. You can retrieve that array by a simple function call to t3lib_div::_GP('tx_myext'); or t3lib_div::_GET('tx_myext'); Please see the Extension API and tutorials in this regard for more guidelines.

Never use getenv()

Always use the t3lib_div::getIndpEnv() function if you can. You can depend on those values, that they will work on any platform TYPO3 supports.

If you really need a variable from HTTP_SERVER_VARS for some reason, you can most likely do so. You may then help us testing the availability of this variable on a variety of systems and suggest to include it as a key in t3lib_div::getIndpEnv().