I came across this error when I was trying to implementing the Assetlib-pro CodeIgniter library for adding assets etc to your project.
A PHP Error was encountered
Message: filemtime() [function.filemtime]: stat failed for
Line Number: 218
Now because of my setup from EukHost in the uk I use plesk as my admin system, which obviously has custom rules etc for how php and apache behave together and how they are setup. For some reason the assetlib code could correctly get to the styles I had defined in the assetlib config file – ‘/assets/’, but I had to manually change a line of code in the assetlib library file.