Patrick Allaert has put together a shell command that can take your PHP4 code and replace its current constructors with PHP5-formatted ones.

It assumes your classes are always declared with the class keyword starting at the beginning of the line and that your files have the .php extension.

It uses a regular expression in a perl command to search through the current directory and look for the ".php" files to replace the "function ClassName" sort of thing with a "function __construct".