diff --git a/src/utils/utf8.php b/src/utils/utf8.php
--- a/src/utils/utf8.php
+++ b/src/utils/utf8.php
@@ -289,8 +289,21 @@
  */
 function phutil_utf8_strlen($string) {
   if (function_exists('utf8_decode')) {
-    return strlen(utf8_decode($string));
-  }
+    // Historically, this was just a call to strlen(utf8_decode($string))
+    // but, since PHP 8.2, that function is deprecated, so this is
+    // the current equivalent.
+    // Note that we cannot just adopt mb_strlen($string, 'ISO-8859-1')
+    // since it's not the exact equivalent as far as we can see.
+    // https://we.phorge.it/T15188
+    return strlen(mb_convert_encoding($string, 'ISO-8859-1'));
+  }
+
+  // TODO: Eventually drop the next function call, since it was
+  // probably just a backward compatibility hack for the lack
+  // of utf8_decode() that - in any case - we should not use it.
+  // Note that - in any case - this replacement probably works since PHP 4:
+  //   strlen( mb_convert_encoding($string, 'ISO-8859-1') )
+  // So it's really probably the case to just drop this case.
   return count(phutil_utf8v($string));
 }