Code Modernisation: Document when the Hash polyfills can be removed.

The Hash extension cannot be disabled as of PHP 7.4. So, while we can't remove these polyfills yet, we can document when we'll be able to.

Props jrf.
Fixes #47698.



git-svn-id: https://develop.svn.wordpress.org/trunk@45638 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/compat.php

@@ -221,6 +221,13 @@ function _mb_strlen( $str, $encoding = null ) {
 	/**
 	 * Compat function to mimic hash_hmac().
 	 *
+	 * The Hash extension is bundled with PHP by default since PHP 5.1.2.
+	 * However, the extension may be explicitly disabled on select servers.
+	 * As of PHP 7.4.0, the Hash extension is a core PHP extension and can no
+	 * longer be disabled.
+	 * I.e. when PHP 7.4.0 becomes the minimum requirement, this polyfill
+	 * and the associated `_hash_hmac()` function can be safely removed.
+	 *
 	 * @ignore
 	 * @since 3.2.0
 	 *
@@ -335,10 +342,15 @@ function _json_decode_object_helper( $data ) {
 	 *
 	 * Compares two strings using the same time whether they're equal or not.
 	 *
-	 * This function was added in PHP 5.6.
-	 *
 	 * Note: It can leak the length of a string when arguments of differing length are supplied.
 	 *
+	 * This function was added in PHP 5.6.
+	 * However, the Hash extension may be explicitly disabled on select servers.
+	 * As of PHP 7.4.0, the Hash extension is a core PHP extension and can no
+	 * longer be disabled.
+	 * I.e. when PHP 7.4.0 becomes the minimum requirement, this polyfill
+	 * can be safely removed.
+	 *
 	 * @since 3.9.2
 	 *
 	 * @param string $a Expected string.