PHP 4 adds a new mechanism to variable substitution in strings. You can now finally access object member variables and elements from multidimensional arrays within strings.
To do so you have to enclose your variables with curly braces with the dollar sign immediately following the opening brace: {$...}
To embed the value of an object member variable into a string you simply write "text {$obj->member} text" while in PHP 3 you had to use something like "text ".$obj->member." text".
This should lead to more readable code, while it may break existing scripts written for PHP 3. But you can easily check for this kind of problem by checking for the character combination {$ in your code and by replacing it with \{$ with your favorite search-and-replace tool.