Description:
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This bug has duplicates with status closed here (for example #18556). But the problem is still continue to survive in the latest PHP.

The problem is case insensitive function name handling in PHP's scripting engine and the native behavior of Turkish language. Turkish language has a letter "I" of which the lowercase is dotless i ("ı"). PHP engine itself running on different locale lowercases and registers user defined functions with its running environment's locale. But while parsing the user's PHP source code and the code itself has a call setlocale(LC_ALL, "tr_TR") then the user defined functions are lowercased with Turkish locale. If the function name contains the letter "I" then it is never matched with the previously registered lowercased one.

For example getImageDir() is registered as getimagedir() but with Turkish locale set by user code it is compared with getımagedir() and the results is "undefined function". I think this is a developer bug because of using locale aware functions for lowercasing native english keywords.

The problem affects function names, class names, method names, ... all incasesensitive staff. And all of them have to be native English.

I made a solution by modifying the source file "php-5.1.1/Zend/zend_operators.c". Instead of using the locale aware "tolower(int)" libc function in zend_str_tolower(), zend_str_tolower_copy() functions, I I wrote down a "tolower_english(int)" equivalent to "tolower()" and called it in zend_str_tolower() / zend_str_tolower() instead of the original one "tolower()".

Function  names, class names, method names are not locale aware staff, so why to use locale aware tolower() libc function? Function names, class names, method names are natively in English locale, aren't they? I don't understand why developers use locale aware functions for native english keywords like function names, class names, method names, ... etc.

I am tired of patching the PHP code with my non-perfect solution.

Reproduce code:
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My solution is not perfect, it may cause unexpected problems. But I use it without problems until now.

It is better to use strictly English locale functions instead of locale aware one for collating natively English strings.

http://www.pro-g.com.tr/php_turkish_bug/zend_operators.c.diff