PHP CSV diff script - php

Question

I am currently writing a simple PHP script/site to save the differences of 2 csv files in a 3rd one.
Currently my only problem is, that I want to ignore a specific row (the customer id) which changes from time to time.
I am writing a newsletter system and I have 2 csv files with customer data, and I am checking, if the email address has changed since the last csv file.
How can I ignore the customer id, I can't just delete the column, because I need that column for later.
Here is my structure:
CSV file 1 (old database):
customerid,sex,name,firstname,zip,email
1,male,smith,will,1234,will.smith#gmail.com
2,male,doe,john,7367,john#doe.com
3,female,doe,anna,7367,anne#doe.com
CSV file 2 (new database):
customerid,sex,name,firstname,zip,email
1,male,smith,will,2224,will.smith#gmail.com
7,male,doe,john,7367,john#gmail.com
20,female,doe,anna,7367,anne#doe.com
As you can see, in the newer file, the customer id for John and Anna Doe changed, this should be ignored.
What shouldn't be ignored, is that the email for John Doe and the zip for Will Smith changed. The second "step" works perfectly.
Here is my full code: http://pastebin.com/bt7Pj3MP (about 30 lines). There are the essential parts:
$file1 = file('2015-07-01.csv', FILE_IGNORE_NEW_LINES);
$file2 = file('2015-07-09.csv', FILE_IGNORE_NEW_LINES);
sort($file1);
sort($file2);
$diff = array_diff($file2, $file1);
array_unshift($diff, $_POST['csv_cols']);
$output = substr(md5(rand()), 0, 5). "-output.csv";
file_put_contents($output, implode(PHP_EOL, $diff));
unlink($file1_name);
unlink($file2_name);
header('Content-Description: Download ' . $output);
header('Content-Type: application/force-download');
header("Content-Type: application/download");
header("Content-Length: " . filesize($output));
header("Content-disposition: attachment; filename=\"" . basename($output) . "\"");
readfile($output);
unlink($output);
exit;