#552 allow_url_fopen considered harmful

[Daniel Drucker]

allow_url_fopen is disabled on many popular web hosting providers (e.g.,
Dreamhost), for security reasons. This means it is impossible to load
external webcals ... ical_parser.php fails at
$ifile = @fopen($filename, "r");

A solution is to use a curl class instead.

[Jim Hu]

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Some php installs are compiled without curl. I suppose which to use could be
settable as an option during install.