Best practice to detect a ClassicPress Install

When developing Plugins or Themes intended to work with ClassicPress, or adding any arbitrary code that should be fired only on ClassicPress installs, you can check if the classicpress_version() function exists using the function_exists() method.

This function is only registered by ClassicPress, and thus if your check returns false, it is not a ClassicPress Install.

An example implementation:

function is_classicpress() {
    if ( function_exists( 'classicpress_version' ) ) {
        return true;
    } else {
        return false;

Then use it like so in your code:

if( true === is_classicpress() ){
    // Do your CP magic.