Paul Gregg has written up an example of what he calls "tail -# file for PHP" (starting from the end of file and moving backwards a given number of lines).

Here I will demonstrate a fairly tuned method of seeking to the end of the file and stepping back to read sufficient lines to the end of the file. If insufficient lines are returned, it incrementally looks back further in the file until it either can look no further, or sufficient lines are returned.

His script meets two goals - reading enough lines in for the request and keeping those lines to a minimum. His code grabs the size of the file and opens the file as a resource to fseek to a certain point and read in the lines from the defined start to finish and push them into an array. You can see the source here and a sample execution here.