Deprecate calling mb_check_encoding() without argument

Part of https://wiki.php.net/rfc/deprecations_php_8_1.

Author: nikic

UPGRADING

@@ -337,6 +337,10 @@ PHP 8.1 UPGRADE NOTES
     favor of date_sun_info().
     RFC: https://wiki.php.net/rfc/deprecations_php_8_1
 
+- Mbstring:
+  . Calling mb_check_encoding() without an argument is deprecated.
+    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
+
 - MySQLi:
   . The mysqli_driver::$driver_version property has been deprecated. The driver
     version is meaningless as it hasn't been updated in more than a decade. Use

ext/mbstring/mbstring.c

@@ -3953,6 +3953,9 @@ PHP_FUNCTION(mb_check_encoding)
 	} else if (input_str) {
 		RETURN_BOOL(php_mb_check_encoding(ZSTR_VAL(input_str), ZSTR_LEN(input_str), encoding));
 	} else {
+		php_error_docref(NULL, E_DEPRECATED,
+			"Calling mb_check_encoding() without argument is deprecated");
+
 		/* FIXME: Actually check all inputs, except $_FILES file content. */
 		RETURN_BOOL(MBSTRG(illegalchars) == 0);
 	}

ext/mbstring/tests/mb_check_encoding_without_arg.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Calling mb_check_encoding() without argument is deprecated
+--EXTENSIONS--
+mbstring
+--FILE--
+<?php
+
+var_dump(mb_check_encoding());
+
+?>
+--EXPECTF--
+Deprecated: mb_check_encoding(): Calling mb_check_encoding() without argument is deprecated in %s on line %d
+bool(true)