PHP 8.5 does not have a convert_cyr_string() function. This function was deprecated in PHP 7.4.0 and completely removed in PHP 8.0.0. Calling it in PHP 8.5 will result in a fatal error (Fatal Error).
Why was it removed?
convert_cyr_string() was only capable of handling a fixed set of legacy single-byte Cyrillic character sets (such as KOI8-R, Windows-1251, etc.). It did not support UTF-8, and if it encountered an input that did not match the declared source encoding, it would silently corrupt the data. Modern PHP recommends using more powerful general-purpose encoding conversion functions.
What should be used in PHP 8.5 instead?
Use mb_convert_encoding() (requires the mbstring extension) or iconv(). Both support UTF-8 and virtually all modern character sets.
Example: Converting legacy Windows-1251 Russian text to UTF-8
<?php
// Assume $legacy_string contains Windows-1251 encoded bytes
$legacy_string = "xCFxF0xE8xE2xE5xF2"; // "Привет" in Windows-1251// Method 1: Using mb_convert_encoding (recommended)
$utf8_string = mb_convert_encoding($legacy_string, "UTF-8", "Windows-1251");
echo $utf8_string; // Output: Привет// Method 2: Using iconv
$utf8_string = iconv("Windows-1251", "UTF-8", $legacy_string);
echo $utf8_string; // Output: Привет
?>
Legacy syntax of convertcyrstring (for reference only)
If you encounter it in very old projects, its original signature and character set parameters were as follows (code below cannot run in PHP 8.5):
// Legacy example only — will throw Fatal Error on PHP 8+
$converted = convert_cyr_string($str, "w", "k"); // Windows-1251 -> KOI8-R
Supported single-character encoding codes:
k = koi8-r
w = windows-1251
i = iso8859-5
a / d = x-cp866
m = x-mac-cyrillic
Summary: In PHP 8.5, simply forget about convert_cyr_string() and use mb_convert_encoding() or iconv() directly.