1st snippet in post "How to Check Whether Other WordPress Plugin is Active or Not"

gistfile1.php

function prfx_check_EDD() {
	// Check if core EDD plugin is not active
	if ( ! class_exists( 'Easy_Digital_Downloads') ) {		
		// Check if plugin active
		if ( is_plugin_active( plugin_basename( __FILE__ ) ) ) {
			// Deactivate plugin
			deactivate_plugins( plugin_basename( __FILE__ ) );
			// Unset activation notice
	 		unset( $_GET[ 'activate' ] );	
			// Add error notice on admin screen
			add_action( 'admin_notices', 'prfx_admin_notice' );		
		}
	}
}
add_action( 'admin_init', 'prfx_check_EDD' );