PHP issues

PHP syntax: to get a value of a variable from url, like: $url = "show_cat.php?catid=".($row["catid"]). The PHP script should be dealt with something like: $var=_REQUEST(“var”). if don’t do like this, you can’t get the value of the variable.
We can’t use reserve variable $PHP_SELF directly in PHP file, we should use $_SERVER['PHP_SELF'] instead. I think some version may support $PHP_SELF directly, I need to know this.
To judge some variable coming from a form of another webpage should use something like “ if (isset($_REQUEST['username']) && isset($_REQUEST['passwd']))”