I think this was a good explanation:
"No, the problem results because lowercase i (in most languages) and uppercase I
(in most languages) are not actually considered to be the upper/lower variant of
the same letter in Turkish. In Turkish, the undotted ı is the lowercase of I,
and the dotted İ is the uppercase of i. If you have a class named Image, it
will break if the locale is changed to turkish because class_exists() function
uses zend_str_tolower(), and changes the case on all classes, because they are
supposed to be case insensitive. Someone else above explained it very well:
"class_exists() function uses zend_str_tolower(). zend_str_tolower() uses
zend_tolower(). zend_tolower() uses _tolower_l() on Windows and tolower() on
other oses. _tolower_l() is not locale aware. tolower() is LC_CTYPE aware."
Edit: Someone else later said the following (I'm wondering if it's true):
"This, practically, can't be fixed. Mainly because there's no way to know if 'I' is uppercase of 'i' or 'ı' since there's not a separate place for Turkish 'I' in code tables. The same holds for 'i' (can't be known if it's lowercase of 'I' or 'İ').
I told 2 years ago and will say it again: PHP should provide a way to turn off case-insensitive function/class name lookup. No good programmer uses this Basic language feature since identifiers are case-sensitive in all real languages like Python, Ruby, C#, Java."