Check for a minimum level of PHP and stop activation if the current host is below this

yourplugin.php

<?php
function check_php_level() {

	$php = '[[your minimum PHP level]]'; // Minimum PHP level required

	/* translators: %1$s: required PHP version, %2$s: current PHP level */
	$message = sprintf( __( 'The [[your plugin name]] plugin requires PHP version %1$s or greater but you are using version %2$s. The plugin has NOT been activated.', '[[your text-domain]]' ), $php, PHP_VERSION );
	$title   = __( 'Plugin Activation Error', '[[your text-domain]]' );

	if ( version_compare( PHP_VERSION, $php, '<' ) ) {
		wp_die( esc_html( $message ), esc_html( $title ) );
	} else {
		return;
	}
}

register_activation_hook( __FILE__, 'check_php_level' );

Simple function, to add to a WordPress plugin, to check for the current level of PHP and fail activation if the current hosting environment is less than the one that's required for the plugin to run.

Replace all content contained within double square brackets before using.