Please firstly ensure that you are running the latest version of the Joomla 1.5 series which is 1.5.26
If this does not solve the issue, it could be due to the fact that you're using a security plugin for you admin panel. If you need to access the admin panel by typing in a key after the URL like so AND it's a number:
www.mysite.com/administrator?12345
Then change the key so that it doesn't use numbers
This error is actually produced by Joomla and not PHP, so as mentioned before, ensure you running the latest version. I would also recommend that you upgrade to Joomla 2.5 or 3.x as 1.5 has some pretty big security flaws.