Microsoft Home Publishing
Microsoft Home Publishing was released in two versions, 99 & 2000. Software usually came bundled with MIX_(Picture_It!) software.
This program confusingly uses the .php extension for its files (for paper projects; e-mail projects have a .mhp extension), an extension commonly associated with the PHP programming language, so any attempts to search or sort these files based only on extension will likely have many false identifications, especially if the computer on which the files are located has been used for web development or hosting, where the PHP language is heavily used. If the files are in text format and contain something that looks like program code or web content, they're probably PHP language, not Microsoft Home Publishing.
PHP files can be opened by later versions of Microsoft Digital Image Suite.