Hack or no Hack?

Recenly i had an error and could not access my ACP, this was not on just 1 site i had it was on all 5 sites i run, after checking i found that all my index.php pages had a line added at the top of the page on line 1
[code]what is supost to be on line 1 is "<?php"what i found was this<iframe src="http]
so is this a hack or a hole for someone to access your site?
[code]what is supost to be on line 1 is "<?php"what i found was this<iframe src="http]
so is this a hack or a hole for someone to access your site?