Are PHP case-sensitive functions if not ...?

I use many functions in my PHP codes like these; UID(), user_getProfileImage() ... etc. I write all my projects on Windows and work well. This is normal, but when I put my project on my server, it gave an error like this:

Fatal error: Call to undefined function UID() in /var/www/vhosts/...

What? Is it undefined ?!

I check all my project files and FTP uploads all the files to the server again, again ... And the same error.

But when I change the name UID()to UID()(both in lib.php and other places where it is used), it works well.

So what's the problem? What is wrong with this server?

Local PHP vers: 5.3.10

PHP server info: http: // ... deleted

Note. I encode all PHP files in "UTF-8 without specification" (as always) using Notepad ++, and it is interesting that another project works well, even uses the same functions and runs on the same server.

Thank.

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UPDATE (and solution);

• Do not use the symbol “I” (metropolitan “i”) in any function name or
• Just use setlocale like this; setlocale(LC_TIME, "tr_TR.UTF-8")// I only need to set the locale time and use this

• If you need LC_ALL, be sure to install back LC_CTYPEin en_US, i.e.:

  setlocale(LC_ALL, "tr_TR.UTF-8"); setlocale(LC_CTYPE, "en_US");