LAMP is an acronym for “Linux, Apache, MySQL, Perl/PHP/Python”.
PHP (or Perl, Python)
LINUX Operating System
The reason they call it a stack is because each level derives off it’s base layer. Your Operating system, Linux, is the base layer. Then Apache, your web daemon sits on top of your OS. Then your database stores all the information served by your web daemon, and PHP (or any P* scripting language) is used to drive and display all the data, and allow for user interaction.