Plugins: Block plugin activation if it requires a higher version of PHP or WordPress.

Introduce `validate_plugin_requirements()` for validating a plugin's WordPress and PHP version requirements.

Introduce `wp_is_wp_compatible()` and `wp_is_php_compatible()` for checking compatibility with the current WordPress or PHP version.

Props afragen, joyously, DrewAPicture, TimothyBlynJacobs, desrosj, flixos90, SergeyBiryukov.
See #43992.

git-svn-id: https://develop.svn.wordpress.org/trunk@44978 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6897,3 +6897,29 @@ function wp_direct_php_update_button() {
 	);
 	echo '</p>';
 }
+
+/**
+ * Checks compatibility with the current WordPress version.
+ *
+ * @since 5.2.0
+ *
+ * @param string $required Minimum required WordPress version.
+ * @return bool True if required version is compatible or empty, false if not.
+ */
+function wp_is_wp_compatible( $required ) {
+	$wp_version = get_bloginfo( 'version' );
+
+	return empty( $required ) || version_compare( $wp_version, $required, '>=' );
+}
+
+/**
+ * Checks compatibility with the current PHP version.
+ *
+ * @since 5.2.0
+ *
+ * @param string $required Minimum required PHP version.
+ * @return bool True if required version is compatible or empty, false if not.
+ */
+function wp_is_php_compatible( $required ) {
+	return empty( $required ) || version_compare( phpversion(), $required, '>=' );
+}