This codes a bit tough to trace since you don't have indentation here.
The CSV is logically a two dimensional array or a collection of collection. So you could just split the readTextFile entries up into a collection. Problem is I have no idea what is in that collection, so I can't really tell you what to do with it.
You can write a simple CSV interpreter by simply splitting the string on the delimiter. A good one would need to also determine enclosure characters since a string in a csv file could include the delimiter character as well. If its something like a CSV of numbers, than a simple tokenizer on the delimiter would be sufficient. Collections can then determine min and max very easily using the Collections.min and Collections.max methods.
header('HTTP/1.1 420 Enhance Your Calm');