Talvez esto, tomado del manual de php
puede ayudar a entender esta funcion ->
Cita: This function is sensible to the current locale, namely the LC_CTYPE category (the default LC_CTYPE category is set from the LANG environment variable or by an explicit LC_CTYPE setting, but it can be overriden by the LC_ALL environment setting). If no locale setting is done in the enironment, the default locale will be C, for which the lowercase/uppercase conversion is based on the default character set of the system: this may convert only ASCII letters, or also ISO-8859-1 letters depending on the system...
Saludos
Franco