Some plugins have compatibility issued with various wordpress versions. So if you have to check the current version of wordpress while activating your plugin and also deny installation in some lower wordpress versions you can do this as :
<?php /* Plugin Name: myPlugin Plugin URI: your url address Description: my first wordpress plugin WordPress admin interface Version: 1.0 Author: you naem Author URI: License: GPLv2 */ register_activation_hook(__FILE__,'firstRunOnActivateFunction'); function firstRunOnActivateFunction() { global $wp_version; // check for wordpress verion 3.5 and lower // if the version is lower than 3.5, installation will be denied If (version_compare($wp_version, "3.5", "<")) { deactivate_plugins(basename(__FILE__)); // Deactivate our plugin wp_die("This plugin requires WordPress version 3.5 or higher."); } } ?>