Quick-fix: Normalize UTF-8 charset slug detection.

In the merge of [58417] the new file was missed. This commit adds the missing required file.

Developed in WordPress#6535
Discussed in https://core.trac.wordpress.org/ticket/61182

Fixes #61182.
Follow-up to [58147].
Props dmsnell, jonsurrell.


git-svn-id: https://develop.svn.wordpress.org/trunk@58148 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/unicode.php

@@ -0,0 +1,51 @@
+<?php
+/**
+ * WordPress API providing Unicode-relevant utilities.
+ *
+ * @since 6.6.0
+ *
+ * @package WordPress
+ */
+
+/**
+ * Indicates if a given slug for a character set represents the UTF-8
+ * text encoding. If not provided, examines the current blog's charset.
+ *
+ * A charset is considered to represent UTF-8 if it is a case-insensitive
+ * match of "UTF-8" with or without the hyphen.
+ *
+ * Example:
+ *
+ *     true  === is_utf8_charset( 'UTF-8' );
+ *     true  === is_utf8_charset( 'utf8' );
+ *     false === is_utf8_charset( 'latin1' );
+ *     false === is_utf8_charset( 'UTF 8' );
+ *
+ *     // Only strings match.
+ *     false === is_utf8_charset( [ 'charset' => 'utf-8' ] );
+ *
+ *     // Without a given charset, it depends on the site option "blog_charset".
+ *     $is_utf8 = is_utf8_charset();
+ *
+ * @since 6.6.0
+ *
+ * @param ?string $blog_charset Slug representing a text character encoding, or "charset".
+ *                              E.g. "UTF-8", "Windows-1252", "ISO-8859-1", "SJIS".
+ * @return bool Whether the slug represents the UTF-8 encoding.
+ */
+function is_utf8_charset( $blog_charset = null ) {
+	$charset_to_examine = $blog_charset ?? get_option( 'blog_charset' );
+
+	/*
+	 * Only valid string values count: the absence of a charset
+	 * does not imply any charset, let alone UTF-8.
+	 */
+	if ( ! is_string( $charset_to_examine ) ) {
+		return false;
+	}
+
+	return (
+		0 === strcasecmp( 'UTF-8', $charset_to_examine ) ||
+		0 === strcasecmp( 'UTF8', $charset_to_examine )
+	);
+}