provided by Star2Billing S.L.

This will not be much comfort to you, but I know of a couple of A2B installs with over 5,000 cards and they've not reported any such problems.
Your web browser is trying to download the PHP code?!? What is the contents of the file if you choose to save it?
I'd love to know what's going on here! My current guess is some strange interaction with your virus scanner or suchlike.

Not necessarily; all useful software performs differently given a different input.

So adjust these setting in the upward direction and retest. See the "Resource Limits" section of php.ini.

I had the exact same issue after adding exactly 5000 new customers. I would get a blank page whenever accessing the CDR Report tool. The source of the problem finally revealed itself after monitoring the /var/log/httpd/error_log;

PHP Fatal error: Allo wed memory size of 8388608 bytes exhausted (tried to allocate 7680 bytes) in /va r/www/html/admin/lib/Smarty/Smarty.class.php on line 1252

Solution was to increase the memory_limit in /etc/php.ini to '32M'. Default for my setup was 8M.

For the record my setup is;

FC6- 2.6.22.9-61.fc6
MySQL- 5.0.27
PHP- 5.1.6
A2billing-1.3.0 + 1.3.2 (Problem occurred on both versions)
Asterisk-1.2.19

I hope this info helps somebody else.

Good luck!

You'll be pleased to know I fixed the cause of the excessive memory usage for the CDR reports a couple of day ago.