Strange Fatal errors in PHP

I use apache,PHP and MySql for my site and have got 2 strange Fatal errors in past few days.

[06-Jan-2015 13:18:09] PHP Fatal error: Call to undefined function Ť () in on line 112
[02-Jan-2015 21:18:15] PHP Fatal error: Cannot instantiate interface XcX–` in € ûq` on line 654

What could cause these? Is someone trying to hack the site?

It's a webapp with 100s of PHP files. I am clueless about what portion of code is causing these errors since these error logs don't say anything about the source.