PHP being a ridiculously easy web scripting language, can be used (and is used) as a great front end tool for any high scale application. In many cases you need to have a back end of Java, C++ etc. but it may not be a great idea to create front end (i.e. web end) using those languages. One of the way to combine these back end with front end would be the use of XSLT transformation.
Your back end can create an XML. Using PHP-XSLT you can create the HTML front end for this application. The original XML will content the variables that you want to show on the front end. XSLT - being a transformation language - would convert the XML to HTML. All three (XML, XSLT and HTML) being markup languages, this process is easy and fast too.
While using Java and C++ are such platforms that will require such transformation for front end, I have used it for .Net. I know .Net already has ASP.Net which can do this but in our requirement, we had to do the transformation from VB.Net to PHP. No matter how weird it may sound, but it was a genuine requirement mainly because of the legacy of the program. I would discuss it in detail sometime.
To use PHP-XSLT transformation refer this link - http://atom.dosimple.ch/docs/html/d1/d3/xslt_8class_8php-source.html