Well, now you're talking about string parsing and variable substitution, not about incoming form variables.
Inside a double-quoted or heredoc string, PHP performs variable substitution. It can parse up to one level of array indexing automatically. Additional levels of nesting require you to either break out of the string and concatenate the array value, or use curly-brace syntax. Personally, I use curly-brace syntax for almost all variable substitutions within strings.
Examples:
$query <<<EOQ
INSERT INTO {$table} VALUES
(NULL,
'{$_POST['user']}', // curly-braces allow you to quote array indexes!
'$_POST[pass]', // without curly-braces, don't quote your index!
'$obj->var') // can access object vars, too!
EOQ;
There's a wealth of information about all the nuances of variable substitution, with examples, in the manual:
http://www.php.net/types.string
Take care,
Nik
http://www.bigaction.org/