I had many PHP warnings in my Drupal logs.
Warning: htmlspecialchars() [function.htmlspecialchars]: Invalid multibyte sequence in argument in check_plain() (Line 1577 of includes/bootstrap.inc).
The reason for this problem was that an UTF-8 string containing multibyte charachers was manipulated with standard (i.e. singlebyte) PHP functions such as
PHP multibyte string functions must be used, see in the PHP reference Multibyte String Functions.
- Use the
umodifier for regex such as
Multibyte problems with UTF-8 can be quite tricky as the may occur only sporadic since UTF-8 is for western countries mostly singlebyte.