The truth is....PHP is a great scalable language...and building large, database driven applications means using database such as postgresql, oracle, sybase, DB2, informix. However the most popular for large scale e-tailers seems to be Oracle, however they are all seeming to find out the wonderful possibilities of making proprietary C/C++ solutions that scale and suit thier needs.
PHP is very secure...when you know how to secure it with special techniques. You can use the built in session functions, or write a database driven one for a larger application. PHP and any other database such as postgre can work faster under larger amounts of pressure, not to mention it can easily take advantage of the hardware once you apply simple tweaks to its ram usage.
Mysql is more for the beginners and smaller applications that do not demand as much power or toughness unlike the larger applications that need something different.
To put it short....any site that currently uses vBulletin is using PHP and Mysql (However I have seen sites port this to other databases such as sybase)
Not to knock JSP or anything...JSP is actually a fairly fast language to use (Java is soo powerful it scares me) and scalability is easily done, however is mainly objetc oriented unlike PHP....where its not completely oriented on objects and more so structured code.
Thats what I have to say as an answer though...