penders - 4:36 pm on Oct 16, 2012 (gmt 0)

I would certainly look at the array functions, as coopster suggests.

Another alternative is to store just one file "data.csv" as the keys of an array (not the values), then step through "local_data.csv" line by line (don't read into memory in its entirety) and check for its presence in the array using isset() - this is much more efficient than using in_array().