Multi-Byte character breaks tempnam functionality

Description

Hi all,

first of all thanks for all the development on PHP. Love the language more than ever. In a recent project I encountered this bug (?):

tempnam() shortens the given prefix after character 63. When passing a multi-byte character in this position, the shortened prefix contains a malformed character. MacOS does not seem to be able to handle that, hence no file is created and a notice is triggered. The PHP docs do not state, that multi-byte strings are not allowed with tempnam().

To reproduce

1. Create a new folder
2. Create script test.php in this folder and paste the following code into it:

<?php
// String concatenation only for highlighting the malicious character
$prefix = 'this_string_pushes_the_multi_byte_character_in_position_______' . 'ä' . 'foobar';
var_dump(tempnam(__DIR__, $prefix));

3. Run php test.php in that directory.

Desired behaviour

It should create a temporary file in this folder and tempnam() should return the path to the file:

string(108) "/Users/benedict/dev/multibyte/this_string_pushes_the_multi_byte_character_in_position_______atWEta7"

Actual behaviour

On MacOS (tested with Sonoma 14.6.1 and Sequoia 15.1) no temporary file is created, a notice is triggered and tempnam() returns false:

PHP Notice:  tempnam(): file created in the system's temporary directory in /Users/benedict/dev/multibyte/test.php on line 16

Notice: tempnam(): file created in the system's temporary directory in /Users/benedict/dev/multibyte/test.php on line 16
bool(false)

When you change the ä in position 63 for anything but a multi-byte character, everything works as expected

If I can provide any further information, please let me know.

PHP Version

PHP 8.3.13

Operating System

MacOS Sonoma 14.6.1

Comienza con el punto de entrada tempnam() y reproduce el comportamiento usando el script test.php y el prefijo multibyte proporcionado en macOS. Compara el tratamiento del límite de prefijo de 63 caracteres para ä y los caracteres de un solo byte; se considera terminado cuando tempnam() crea el archivo en la carpeta solicitada y devuelve su ruta sin un notice.